monitoring database activity database activity monitoring A database administrator frequently wonders, What is the system doing right now? This chapter discusses how to find that out. Several tools are available for monitoring database activity and analyzing performance. Most of this chapter is devoted to describing PostgreSQL's cumulative statistics system, but one should not neglect regular Unix monitoring programs such as ps, top, iostat, and vmstat. Also, once one has identified a poorly-performing query, further investigation might be needed using PostgreSQL's EXPLAIN command. discusses EXPLAIN and other methods for understanding the behavior of an individual query. ps to monitor activity On most Unix platforms, PostgreSQL modifies its command title as reported by ps, so that individual server processes can readily be identified. A sample display is $ ps auxww | grep ^postgres postgres 15551 0.0 0.1 57536 7132 pts/0 S 18:02 0:00 postgres -i postgres 15554 0.0 0.0 57536 1184 ? Ss 18:02 0:00 postgres: background writer postgres 15555 0.0 0.0 57536 916 ? Ss 18:02 0:00 postgres: checkpointer postgres 15556 0.0 0.0 57536 916 ? Ss 18:02 0:00 postgres: walwriter postgres 15557 0.0 0.0 58504 2244 ? Ss 18:02 0:00 postgres: autovacuum launcher postgres 15582 0.0 0.0 58772 3080 ? Ss 18:04 0:00 postgres: joe runbug 127.0.0.1 idle postgres 15606 0.0 0.0 58772 3052 ? Ss 18:07 0:00 postgres: tgl regression [local] SELECT waiting postgres 15610 0.0 0.0 58772 3056 ? Ss 18:07 0:00 postgres: tgl regression [local] idle in transaction (The appropriate invocation of ps varies across different platforms, as do the details of what is shown. This example is from a recent Linux system.) The first process listed here is the primary server process. The command arguments shown for it are the same ones used when it was launched. The next four processes are background worker processes automatically launched by the primary process. (The autovacuum launcher process will not be present if you have set the system not to run autovacuum.) Each of the remaining processes is a server process handling one client connection. Each such process sets its command line display in the form postgres: user database host activity The user, database, and (client) host items remain the same for the life of the client connection, but the activity indicator changes. The activity can be idle (i.e., waiting for a client command), idle in transaction (waiting for client inside a BEGIN block), or a command type name such as SELECT. Also, waiting is appended if the server process is presently waiting on a lock held by another session. In the above example we can infer that process 15606 is waiting for process 15610 to complete its transaction and thereby release some lock. (Process 15610 must be the blocker, because there is no other active session. In more complicated cases it would be necessary to look into the pg_locks system view to determine who is blocking whom.) If has been configured the cluster name will also be shown in ps output: $ psql -c 'SHOW cluster_name' cluster_name -------------- server1 (1 row) $ ps aux|grep server1 postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: server1: background writer ... If you have turned off then the activity indicator is not updated; the process title is set only once when a new process is launched. On some platforms this saves a measurable amount of per-command overhead; on others it's insignificant. Solaris requires special handling. You must use /usr/ucb/ps, rather than /bin/ps. You also must use two w flags, not just one. In addition, your original invocation of the postgres command must have a shorter ps status display than that provided by each server process. If you fail to do all three things, the ps output for each server process will be the original postgres command line. statistics PostgreSQL's cumulative statistics system supports collection and reporting of information about server activity. Presently, accesses to tables and indexes in both disk-block and individual-row terms are counted. The total number of rows in each table, and information about vacuum and analyze actions for each table are also counted. If enabled, calls to user-defined functions and the total time spent in each one are counted as well. PostgreSQL also supports reporting dynamic information about exactly what is going on in the system right now, such as the exact command currently being executed by other server processes, and which other connections exist in the system. This facility is independent of the cumulative statistics system. Since collection of statistics adds some overhead to query execution, the system can be configured to collect or not collect information. This is controlled by configuration parameters that are normally set in postgresql.conf. (See for details about setting configuration parameters.) The parameter enables monitoring of the current command being executed by any server process. The parameter enables monitoring of cost-based vacuum delay. The parameter controls whether cumulative statistics are collected about table and index accesses. The parameter enables tracking of usage of user-defined functions. The parameter enables monitoring of block read, write, extend, and fsync times. The parameter enables monitoring of WAL read, write and fsync times. Normally these parameters are set in postgresql.conf so that they apply to all server processes, but it is possible to turn them on or off in individual sessions using the command. (To prevent ordinary users from hiding their activity from the administrator, only superusers are allowed to change these parameters with SET.) Cumulative statistics are collected in shared memory. Every PostgreSQL process collects statistics locally, then updates the shared data at appropriate intervals. When a server, including a physical replica, shuts down cleanly, a permanent copy of the statistics data is stored in the pg_stat subdirectory, so that statistics can be retained across server restarts. In contrast, when starting from an unclean shutdown (e.g., after an immediate shutdown, a server crash, starting from a base backup, and point-in-time recovery), all statistics counters are reset. Several predefined views, listed in , are available to show the current state of the system. There are also several other views, listed in , available to show the accumulated statistics. Alternatively, one can build custom views using the underlying cumulative statistics functions, as discussed in . When using the cumulative statistics views and functions to monitor collected data, it is important to realize that the information does not update instantaneously. Each individual server process flushes out accumulated statistics to shared memory just before going idle, but not more frequently than once per PGSTAT_MIN_INTERVAL milliseconds (1 second unless altered while building the server); so a query or transaction still in progress does not affect the displayed totals and the displayed information lags behind actual activity. However, current-query information collected by track_activities is always up-to-date. Another important point is that when a server process is asked to display any of the accumulated statistics, accessed values are cached until the end of its current transaction in the default configuration. So the statistics will show static information as long as you continue the current transaction. Similarly, information about the current queries of all sessions is collected when any such information is first requested within a transaction, and the same information will be displayed throughout the transaction. This is a feature, not a bug, because it allows you to perform several queries on the statistics and correlate the results without worrying that the numbers are changing underneath you. When analyzing statistics interactively, or with expensive queries, the time delta between accesses to individual statistics can lead to significant skew in the cached statistics. To minimize skew, stats_fetch_consistency can be set to snapshot, at the price of increased memory usage for caching not-needed statistics data. Conversely, if it's known that statistics are only accessed once, caching accessed statistics is unnecessary and can be avoided by setting stats_fetch_consistency to none. You can invoke pg_stat_clear_snapshot() to discard the current transaction's statistics snapshot or cached values (if any). The next use of statistical information will (when in snapshot mode) cause a new snapshot to be built or (when in cache mode) accessed statistics to be cached. A transaction can also see its own statistics (not yet flushed out to the shared memory statistics) in the views pg_stat_xact_all_tables, pg_stat_xact_sys_tables, pg_stat_xact_user_tables, and pg_stat_xact_user_functions. These numbers do not act as stated above; instead they update continuously throughout the transaction. Some of the information in the dynamic statistics views shown in is security restricted. Ordinary users can only see all the information about their own sessions (sessions belonging to a role that they are a member of). In rows about other sessions, many columns will be null. Note, however, that the existence of a session and its general properties such as its sessions user and database are visible to all users. Superusers and roles with privileges of built-in role pg_read_all_stats can see all the information about all sessions. View Name Description pg_stat_activity pg_stat_activity One row per server process, showing information related to the current activity of that process, such as state and current query. See pg_stat_activity for details. pg_stat_replicationpg_stat_replication One row per WAL sender process, showing statistics about replication to that sender's connected standby server. See pg_stat_replication for details. pg_stat_wal_receiverpg_stat_wal_receiver Only one row, showing statistics about the WAL receiver from that receiver's connected server. See pg_stat_wal_receiver for details. pg_stat_recovery_prefetchpg_stat_recovery_prefetch Only one row, showing statistics about blocks prefetched during recovery. See pg_stat_recovery_prefetch for details. pg_stat_subscriptionpg_stat_subscription At least one row per subscription, showing information about the subscription workers. See pg_stat_subscription for details. pg_stat_sslpg_stat_ssl One row per connection (regular and replication), showing information about SSL used on this connection. See pg_stat_ssl for details. pg_stat_gssapipg_stat_gssapi One row per connection (regular and replication), showing information about GSSAPI authentication and encryption used on this connection. See pg_stat_gssapi for details. pg_stat_progress_analyzepg_stat_progress_analyze One row for each backend (including autovacuum worker processes) running ANALYZE, showing current progress. See . pg_stat_progress_create_indexpg_stat_progress_create_index One row for each backend running CREATE INDEX or REINDEX, showing current progress. See . pg_stat_progress_vacuumpg_stat_progress_vacuum One row for each backend (including autovacuum worker processes) running VACUUM, showing current progress. See . pg_stat_progress_clusterpg_stat_progress_cluster One row for each backend running CLUSTER or VACUUM FULL, showing current progress. See . pg_stat_progress_basebackuppg_stat_progress_basebackup One row for each WAL sender process streaming a base backup, showing current progress. See . pg_stat_progress_copypg_stat_progress_copy One row for each backend running COPY, showing current progress. See . View Name Description pg_stat_archiverpg_stat_archiver One row only, showing statistics about the WAL archiver process's activity. See pg_stat_archiver for details. pg_stat_bgwriterpg_stat_bgwriter One row only, showing statistics about the background writer process's activity. See pg_stat_bgwriter for details. pg_stat_checkpointerpg_stat_checkpointer One row only, showing statistics about the checkpointer process's activity. See pg_stat_checkpointer for details. pg_stat_databasepg_stat_database One row per database, showing database-wide statistics. See pg_stat_database for details. pg_stat_database_conflictspg_stat_database_conflicts One row per database, showing database-wide statistics about query cancels due to conflict with recovery on standby servers. See pg_stat_database_conflicts for details. pg_stat_iopg_stat_io One row for each combination of backend type, context, and target object containing cluster-wide I/O statistics. See pg_stat_io for details. pg_stat_replication_slotspg_stat_replication_slots One row per replication slot, showing statistics about the replication slot's usage. See pg_stat_replication_slots for details. pg_stat_slrupg_stat_slru One row per SLRU, showing statistics of operations. See pg_stat_slru for details. pg_stat_subscription_statspg_stat_subscription_stats One row per subscription, showing statistics about errors and conflicts. See pg_stat_subscription_stats for details. pg_stat_walpg_stat_wal One row only, showing statistics about WAL activity. See pg_stat_wal for details. pg_stat_all_tablespg_stat_all_tables One row for each table in the current database, showing statistics about accesses to that specific table. See pg_stat_all_tables for details. pg_stat_sys_tablespg_stat_sys_tables Same as pg_stat_all_tables, except that only system tables are shown. pg_stat_user_tablespg_stat_user_tables Same as pg_stat_all_tables, except that only user tables are shown. pg_stat_xact_all_tablespg_stat_xact_all_tables Similar to pg_stat_all_tables, but counts actions taken so far within the current transaction (which are not yet included in pg_stat_all_tables and related views). The columns for numbers of live and dead rows and vacuum and analyze actions are not present in this view. pg_stat_xact_sys_tablespg_stat_xact_sys_tables Same as pg_stat_xact_all_tables, except that only system tables are shown. pg_stat_xact_user_tablespg_stat_xact_user_tables Same as pg_stat_xact_all_tables, except that only user tables are shown. pg_stat_all_indexespg_stat_all_indexes One row for each index in the current database, showing statistics about accesses to that specific index. See pg_stat_all_indexes for details. pg_stat_sys_indexespg_stat_sys_indexes Same as pg_stat_all_indexes, except that only indexes on system tables are shown. pg_stat_user_indexespg_stat_user_indexes Same as pg_stat_all_indexes, except that only indexes on user tables are shown. pg_stat_user_functionspg_stat_user_functions One row for each tracked function, showing statistics about executions of that function. See pg_stat_user_functions for details. pg_stat_xact_user_functionspg_stat_xact_user_functions Similar to pg_stat_user_functions, but counts only calls during the current transaction (which are not yet included in pg_stat_user_functions). pg_statio_all_tablespg_statio_all_tables One row for each table in the current database, showing statistics about I/O on that specific table. See pg_statio_all_tables for details. pg_statio_sys_tablespg_statio_sys_tables Same as pg_statio_all_tables, except that only system tables are shown. pg_statio_user_tablespg_statio_user_tables Same as pg_statio_all_tables, except that only user tables are shown. pg_statio_all_indexespg_statio_all_indexes One row for each index in the current database, showing statistics about I/O on that specific index. See pg_statio_all_indexes for details. pg_statio_sys_indexespg_statio_sys_indexes Same as pg_statio_all_indexes, except that only indexes on system tables are shown. pg_statio_user_indexespg_statio_user_indexes Same as pg_statio_all_indexes, except that only indexes on user tables are shown. pg_statio_all_sequencespg_statio_all_sequences One row for each sequence in the current database, showing statistics about I/O on that specific sequence. See pg_statio_all_sequences for details. pg_statio_sys_sequencespg_statio_sys_sequences Same as pg_statio_all_sequences, except that only system sequences are shown. (Presently, no system sequences are defined, so this view is always empty.) pg_statio_user_sequencespg_statio_user_sequences Same as pg_statio_all_sequences, except that only user sequences are shown. The per-index statistics are particularly useful to determine which indexes are being used and how effective they are. The pg_stat_io and pg_statio_ set of views are useful for determining the effectiveness of the buffer cache. They can be used to calculate a cache hit ratio. Note that while PostgreSQL's I/O statistics capture most instances in which the kernel was invoked in order to perform I/O, they do not differentiate between data which had to be fetched from disk and that which already resided in the kernel page cache. Users are advised to use the PostgreSQL statistics views in combination with operating system utilities for a more complete picture of their database's I/O performance. pg_stat_activity The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process. Column Type Description datid oid OID of the database this backend is connected to datname name Name of the database this backend is connected to pid integer Process ID of this backend leader_pid integer Process ID of the parallel group leader if this process is a parallel query worker, or process ID of the leader apply worker if this process is a parallel apply worker. NULL indicates that this process is a parallel group leader or leader apply worker, or does not participate in any parallel operation. usesysid oid OID of the user logged into this backend usename name Name of the user logged into this backend application_name text Name of the application that is connected to this backend client_addr inet IP address of the client connected to this backend. If this field is null, it indicates either that the client is connected via a Unix socket on the server machine or that this is an internal process such as autovacuum. client_hostname text Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when is enabled. client_port integer TCP port number that the client is using for communication with this backend, or -1 if a Unix socket is used. If this field is null, it indicates that this is an internal server process. backend_start timestamp with time zone Time when this process was started. For client backends, this is the time the client connected to the server. xact_start timestamp with time zone Time when this process' current transaction was started, or null if no transaction is active. If the current query is the first of its transaction, this column is equal to the query_start column. query_start timestamp with time zone Time when the currently active query was started, or if state is not active, when the last query was started state_change timestamp with time zone Time when the state was last changed wait_event_type text The type of event for which the backend is waiting, if any; otherwise NULL. See . wait_event text Wait event name if backend is currently waiting, otherwise NULL. See through . state text Current overall state of this backend. Possible values are: starting: The backend is in initial startup. Client authentication is performed during this phase. active: The backend is executing a query. idle: The backend is waiting for a new client command. idle in transaction: The backend is in a transaction, but is not currently executing a query. idle in transaction (aborted): This state is similar to idle in transaction, except one of the statements in the transaction caused an error. fastpath function call: The backend is executing a fast-path function. disabled: This state is reported if is disabled in this backend. backend_xid xid Top-level transaction identifier of this backend, if any; see . backend_xmin xid The current backend's xmin horizon. query_id bigint Identifier of this backend's most recent query. If state is active this field shows the identifier of the currently executing query. In all other states, it shows the identifier of last query that was executed. Query identifiers are not computed by default so this field will be null unless parameter is enabled or a third-party module that computes query identifiers is configured. query text Text of this backend's most recent query. If state is active this field shows the currently executing query. In all other states, it shows the last query that was executed. By default the query text is truncated at 1024 bytes; this value can be changed via the parameter . backend_type text Type of current backend. Possible types are autovacuum launcher, autovacuum worker, logical replication launcher, logical replication worker, parallel worker, background writer, client backend, checkpointer, archiver, standalone backend, startup, walreceiver, walsender, walwriter and walsummarizer. In addition, background workers registered by extensions may have additional types. The wait_event and state columns are independent. If a backend is in the active state, it may or may not be waiting on some event. If the state is active and wait_event is non-null, it means that a query is being executed, but is being blocked somewhere in the system. To keep the reporting overhead low, the system does not attempt to synchronize different aspects of activity data for a backend. As a result, ephemeral discrepancies may exist between the view's columns. Wait Event Type Description Activity The server process is idle. This event type indicates a process waiting for activity in its main processing loop. wait_event will identify the specific wait point; see . BufferPin The server process is waiting for exclusive access to a data buffer. Buffer pin waits can be protracted if another process holds an open cursor that last read data from the buffer in question. See . Client The server process is waiting for activity on a socket connected to a user application. Thus, the server expects something to happen that is independent of its internal processes. wait_event will identify the specific wait point; see . Extension The server process is waiting for some condition defined by an extension module. See . InjectionPoint The server process is waiting for an injection point to reach an outcome defined in a test. See for more details. This type has no predefined wait points. IO The server process is waiting for an I/O operation to complete. wait_event will identify the specific wait point; see . IPC The server process is waiting for some interaction with another server process. wait_event will identify the specific wait point; see . Lock The server process is waiting for a heavyweight lock. Heavyweight locks, also known as lock manager locks or simply locks, primarily protect SQL-visible objects such as tables. However, they are also used to ensure mutual exclusion for certain internal operations such as relation extension. wait_event will identify the type of lock awaited; see . LWLock The server process is waiting for a lightweight lock. Most such locks protect a particular data structure in shared memory. wait_event will contain a name identifying the purpose of the lightweight lock. (Some locks have specific names; others are part of a group of locks each with a similar purpose.) See . Timeout The server process is waiting for a timeout to expire. wait_event will identify the specific wait point; see . &wait_event_types; Here are examples of how wait events can be viewed: SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event is NOT NULL; pid | wait_event_type | wait_event ------+-----------------+------------ 2540 | Lock | relation 6644 | LWLock | ProcArray (2 rows) SELECT a.pid, a.wait_event, w.description FROM pg_stat_activity a JOIN pg_wait_events w ON (a.wait_event_type = w.type AND a.wait_event = w.name) WHERE a.wait_event is NOT NULL and a.state = 'active'; -[ RECORD 1 ]------------------------------------------------------&zwsp;------------ pid | 686674 wait_event | WALInitSync description | Waiting for a newly initialized WAL file to reach durable storage Extensions can add Extension, InjectionPoint, and LWLock events to the lists shown in and . In some cases, the name of an LWLock assigned by an extension will not be available in all server processes. It might be reported as just extension rather than the extension-assigned name. pg_stat_replication The pg_stat_replication view will contain one row per WAL sender process, showing statistics about replication to that sender's connected standby server. Only directly connected standbys are listed; no information is available about downstream standby servers. Column Type Description pid integer Process ID of a WAL sender process usesysid oid OID of the user logged into this WAL sender process usename name Name of the user logged into this WAL sender process application_name text Name of the application that is connected to this WAL sender client_addr inet IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine. client_hostname text Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when is enabled. client_port integer TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used backend_start timestamp with time zone Time when this process was started, i.e., when the client connected to this WAL sender backend_xmin xid This standby's xmin horizon reported by . state text Current WAL sender state. Possible values are: startup: This WAL sender is starting up. catchup: This WAL sender's connected standby is catching up with the primary. streaming: This WAL sender is streaming changes after its connected standby server has caught up with the primary. backup: This WAL sender is sending a backup. stopping: This WAL sender is stopping. sent_lsn pg_lsn Last write-ahead log location sent on this connection write_lsn pg_lsn Last write-ahead log location written to disk by this standby server flush_lsn pg_lsn Last write-ahead log location flushed to disk by this standby server replay_lsn pg_lsn Last write-ahead log location replayed into the database on this standby server write_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written it (but not yet flushed it or applied it). This can be used to gauge the delay that synchronous_commit level remote_write incurred while committing if this server was configured as a synchronous standby. flush_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written and flushed it (but not yet applied it). This can be used to gauge the delay that synchronous_commit level on incurred while committing if this server was configured as a synchronous standby. replay_lag interval Time elapsed between flushing recent WAL locally and receiving notification that this standby server has written, flushed and applied it. This can be used to gauge the delay that synchronous_commit level remote_apply incurred while committing if this server was configured as a synchronous standby. sync_priority integer Priority of this standby server for being chosen as the synchronous standby in a priority-based synchronous replication. This has no effect in a quorum-based synchronous replication. sync_state text Synchronous state of this standby server. Possible values are: async: This standby server is asynchronous. potential: This standby server is now asynchronous, but can potentially become synchronous if one of current synchronous ones fails. sync: This standby server is synchronous. quorum: This standby server is considered as a candidate for quorum standbys. reply_time timestamp with time zone Send time of last reply message received from standby server The lag times reported in the pg_stat_replication view are measurements of the time taken for recent WAL to be written, flushed and replayed and for the sender to know about it. These times represent the commit delay that was (or would have been) introduced by each synchronous commit level, if the remote server was configured as a synchronous standby. For an asynchronous standby, the replay_lag column approximates the delay before recent transactions became visible to queries. If the standby server has entirely caught up with the sending server and there is no more WAL activity, the most recently measured lag times will continue to be displayed for a short time and then show NULL. Lag times work automatically for physical replication. Logical decoding plugins may optionally emit tracking messages; if they do not, the tracking mechanism will simply display NULL lag. The reported lag times are not predictions of how long it will take for the standby to catch up with the sending server assuming the current rate of replay. Such a system would show similar times while new WAL is being generated, but would differ when the sender becomes idle. In particular, when the standby has caught up completely, pg_stat_replication shows the time taken to write, flush and replay the most recent reported WAL location rather than zero as some users might expect. This is consistent with the goal of measuring synchronous commit and transaction visibility delays for recent write transactions. To reduce confusion for users expecting a different model of lag, the lag columns revert to NULL after a short time on a fully replayed idle system. Monitoring systems should choose whether to represent this as missing data, zero or continue to display the last known value. pg_stat_replication_slots The pg_stat_replication_slots view will contain one row per logical replication slot, showing statistics about its usage. Column Type Description slot_name text A unique, cluster-wide identifier for the replication slot spill_txns bigint Number of transactions spilled to disk once the memory used by logical decoding to decode changes from WAL has exceeded logical_decoding_work_mem. The counter gets incremented for both top-level transactions and subtransactions. spill_count bigint Number of times transactions were spilled to disk while decoding changes from WAL for this slot. This counter is incremented each time a transaction is spilled, and the same transaction may be spilled multiple times. spill_bytes bigint Amount of decoded transaction data spilled to disk while performing decoding of changes from WAL for this slot. This and other spill counters can be used to gauge the I/O which occurred during logical decoding and allow tuning logical_decoding_work_mem. stream_txns bigint Number of in-progress transactions streamed to the decoding output plugin after the memory used by logical decoding to decode changes from WAL for this slot has exceeded logical_decoding_work_mem. Streaming only works with top-level transactions (subtransactions can't be streamed independently), so the counter is not incremented for subtransactions. stream_countbigint Number of times in-progress transactions were streamed to the decoding output plugin while decoding changes from WAL for this slot. This counter is incremented each time a transaction is streamed, and the same transaction may be streamed multiple times. stream_bytesbigint Amount of transaction data decoded for streaming in-progress transactions to the decoding output plugin while decoding changes from WAL for this slot. This and other streaming counters for this slot can be used to tune logical_decoding_work_mem. total_txns bigint Number of decoded transactions sent to the decoding output plugin for this slot. This counts top-level transactions only, and is not incremented for subtransactions. Note that this includes the transactions that are streamed and/or spilled. total_bytesbigint Amount of transaction data decoded for sending transactions to the decoding output plugin while decoding changes from WAL for this slot. Note that this includes data that is streamed and/or spilled. stats_reset timestamp with time zone Time at which these statistics were last reset pg_stat_wal_receiver The pg_stat_wal_receiver view will contain only one row, showing statistics about the WAL receiver from that receiver's connected server. Column Type Description pid integer Process ID of the WAL receiver process status text Activity status of the WAL receiver process receive_start_lsn pg_lsn First write-ahead log location used when WAL receiver is started receive_start_tli integer First timeline number used when WAL receiver is started written_lsn pg_lsn Last write-ahead log location already received and written to disk, but not flushed. This should not be used for data integrity checks. flushed_lsn pg_lsn Last write-ahead log location already received and flushed to disk, the initial value of this field being the first log location used when WAL receiver is started received_tli integer Timeline number of last write-ahead log location received and flushed to disk, the initial value of this field being the timeline number of the first log location used when WAL receiver is started last_msg_send_time timestamp with time zone Send time of last message received from origin WAL sender last_msg_receipt_time timestamp with time zone Receipt time of last message received from origin WAL sender latest_end_lsn pg_lsn Last write-ahead log location reported to origin WAL sender latest_end_time timestamp with time zone Time of last write-ahead log location reported to origin WAL sender slot_name text Replication slot name used by this WAL receiver sender_host text Host of the PostgreSQL instance this WAL receiver is connected to. This can be a host name, an IP address, or a directory path if the connection is via Unix socket. (The path case can be distinguished because it will always be an absolute path, beginning with /.) sender_port integer Port number of the PostgreSQL instance this WAL receiver is connected to. conninfo text Connection string used by this WAL receiver, with security-sensitive fields obfuscated. pg_stat_recovery_prefetch The pg_stat_recovery_prefetch view will contain only one row. The columns wal_distance, block_distance and io_depth show current values, and the other columns show cumulative counters that can be reset with the pg_stat_reset_shared function. Column Type Description stats_reset timestamp with time zone Time at which these statistics were last reset prefetch bigint Number of blocks prefetched because they were not in the buffer pool hit bigint Number of blocks not prefetched because they were already in the buffer pool skip_init bigint Number of blocks not prefetched because they would be zero-initialized skip_new bigint Number of blocks not prefetched because they didn't exist yet skip_fpw bigint Number of blocks not prefetched because a full page image was included in the WAL skip_rep bigint Number of blocks not prefetched because they were already recently prefetched wal_distance int How many bytes ahead the prefetcher is looking block_distance int How many blocks ahead the prefetcher is looking io_depth int How many prefetches have been initiated but are not yet known to have completed pg_stat_subscription Column Type Description subid oid OID of the subscription subname name Name of the subscription worker_type text Type of the subscription worker process. Possible types are apply, parallel apply, and table synchronization. pid integer Process ID of the subscription worker process leader_pid integer Process ID of the leader apply worker if this process is a parallel apply worker; NULL if this process is a leader apply worker or a table synchronization worker relid oid OID of the relation that the worker is synchronizing; NULL for the leader apply worker and parallel apply workers received_lsn pg_lsn Last write-ahead log location received, the initial value of this field being 0; NULL for parallel apply workers last_msg_send_time timestamp with time zone Send time of last message received from origin WAL sender; NULL for parallel apply workers last_msg_receipt_time timestamp with time zone Receipt time of last message received from origin WAL sender; NULL for parallel apply workers latest_end_lsn pg_lsn Last write-ahead log location reported to origin WAL sender; NULL for parallel apply workers latest_end_time timestamp with time zone Time of last write-ahead log location reported to origin WAL sender; NULL for parallel apply workers pg_stat_subscription_stats The pg_stat_subscription_stats view will contain one row per subscription. Column Type Description subid oid OID of the subscription subname name Name of the subscription apply_error_count bigint Number of times an error occurred while applying changes. Note that any conflict resulting in an apply error will be counted in both apply_error_count and the corresponding conflict count (e.g., confl_*). sync_error_count bigint Number of times an error occurred during the initial table synchronization confl_insert_exists bigint Number of times a row insertion violated a NOT DEFERRABLE unique constraint during the application of changes. See for details about this conflict. confl_update_origin_differs bigint Number of times an update was applied to a row that had been previously modified by another source during the application of changes. See for details about this conflict. confl_update_exists bigint Number of times that an updated row value violated a NOT DEFERRABLE unique constraint during the application of changes. See for details about this conflict. confl_update_missing bigint Number of times the tuple to be updated was not found during the application of changes. See for details about this conflict. confl_delete_origin_differs bigint Number of times a delete operation was applied to row that had been previously modified by another source during the application of changes. See for details about this conflict. confl_delete_missing bigint Number of times the tuple to be deleted was not found during the application of changes. See for details about this conflict. confl_multiple_unique_conflicts bigint Number of times a row insertion or an updated row values violated multiple NOT DEFERRABLE unique constraints during the application of changes. See for details about this conflict. stats_reset timestamp with time zone Time at which these statistics were last reset pg_stat_ssl The pg_stat_ssl view will contain one row per backend or WAL sender process, showing statistics about SSL usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. Column Type Description pid integer Process ID of a backend or WAL sender process ssl boolean True if SSL is used on this connection version text Version of SSL in use, or NULL if SSL is not in use on this connection cipher text Name of SSL cipher in use, or NULL if SSL is not in use on this connection bits integer Number of bits in the encryption algorithm used, or NULL if SSL is not used on this connection client_dn text Distinguished Name (DN) field from the client certificate used, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated if the DN field is longer than NAMEDATALEN (64 characters in a standard build). client_serial numeric Serial number of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. The combination of certificate serial number and certificate issuer uniquely identifies a certificate (unless the issuer erroneously reuses serial numbers). issuer_dn text DN of the issuer of the client certificate, or NULL if no client certificate was supplied or if SSL is not in use on this connection. This field is truncated like client_dn. pg_stat_gssapi The pg_stat_gssapi view will contain one row per backend, showing information about GSSAPI usage on this connection. It can be joined to pg_stat_activity or pg_stat_replication on the pid column to get more details about the connection. Column Type Description pid integer Process ID of a backend gss_authenticated boolean True if GSSAPI authentication was used for this connection principal text Principal used to authenticate this connection, or NULL if GSSAPI was not used to authenticate this connection. This field is truncated if the principal is longer than NAMEDATALEN (64 characters in a standard build). encrypted boolean True if GSSAPI encryption is in use on this connection credentials_delegated boolean True if GSSAPI credentials were delegated on this connection. pg_stat_archiver The pg_stat_archiver view will always have a single row, containing data about the archiver process of the cluster. Column Type Description archived_count bigint Number of WAL files that have been successfully archived last_archived_wal text Name of the WAL file most recently successfully archived last_archived_time timestamp with time zone Time of the most recent successful archive operation failed_count bigint Number of failed attempts for archiving WAL files last_failed_wal text Name of the WAL file of the most recent failed archival operation last_failed_time timestamp with time zone Time of the most recent failed archival operation stats_reset timestamp with time zone Time at which these statistics were last reset Normally, WAL files are archived in order, oldest to newest, but that is not guaranteed, and does not hold under special circumstances like when promoting a standby or after crash recovery. Therefore it is not safe to assume that all files older than last_archived_wal have also been successfully archived. pg_stat_io The pg_stat_io view will contain one row for each combination of backend type, target I/O object, and I/O context, showing cluster-wide I/O statistics. Combinations which do not make sense are omitted. Currently, I/O on relations (e.g. tables, indexes) and WAL activity are tracked. However, relation I/O which bypasses shared buffers (e.g. when moving a table from one tablespace to another) is currently not tracked. Column Type Description backend_type text Type of backend (e.g. background worker, autovacuum worker). See pg_stat_activity for more information on backend_types. Some backend_types do not accumulate I/O operation statistics and will not be included in the view. object text Target object of an I/O operation. Possible values are: relation: Permanent relations. temp relation: Temporary relations. wal: Write Ahead Logs. context text The context of an I/O operation. Possible values are: normal: The default or standard context for a type of I/O operation. For example, by default, relation data is read into and written out from shared buffers. Thus, reads and writes of relation data to and from shared buffers are tracked in context normal. init: I/O operations performed while creating the WAL segments are tracked in context init. vacuum: I/O operations performed outside of shared buffers while vacuuming and analyzing permanent relations. Temporary table vacuums use the same local buffer pool as other temporary table I/O operations and are tracked in context normal. bulkread: Certain large read I/O operations done outside of shared buffers, for example, a sequential scan of a large table. bulkwrite: Certain large write I/O operations done outside of shared buffers, such as COPY. reads bigint Number of read operations. read_bytes numeric The total size of read operations in bytes. read_time double precision Time spent waiting for read operations in milliseconds (if is enabled and object is not wal, or if is enabled and object is wal, otherwise zero) writes bigint Number of write operations. write_bytes numeric The total size of write operations in bytes. write_time double precision Time spent waiting for write operations in milliseconds (if is enabled and object is not wal, or if is enabled and object is wal, otherwise zero) writebacks bigint Number of units of size BLCKSZ (typically 8kB) which the process requested the kernel write out to permanent storage. writeback_time double precision Time spent waiting for writeback operations in milliseconds (if is enabled, otherwise zero). This includes the time spent queueing write-out requests and, potentially, the time spent to write out the dirty data. extends bigint Number of relation extend operations. extend_bytes numeric The total size of relation extend operations in bytes. extend_time double precision Time spent waiting for extend operations in milliseconds. (if is enabled and object is not wal, or if is enabled and object is wal, otherwise zero) hits bigint The number of times a desired block was found in a shared buffer. evictions bigint Number of times a block has been written out from a shared or local buffer in order to make it available for another use. In context normal, this counts the number of times a block was evicted from a buffer and replaced with another block. In contexts bulkwrite, bulkread, and vacuum, this counts the number of times a block was evicted from shared buffers in order to add the shared buffer to a separate, size-limited ring buffer for use in a bulk I/O operation. reuses bigint The number of times an existing buffer in a size-limited ring buffer outside of shared buffers was reused as part of an I/O operation in the bulkread, bulkwrite, or vacuum contexts. fsyncs bigint Number of fsync calls. These are only tracked in context normal. fsync_time double precision Time spent waiting for fsync operations in milliseconds (if is enabled and object is not wal, or if is enabled and object is wal, otherwise zero) stats_reset timestamp with time zone Time at which these statistics were last reset. Some backend types never perform I/O operations on some I/O objects and/or in some I/O contexts. These rows are omitted from the view. For example, the checkpointer does not checkpoint temporary tables, so there will be no rows for backend_type checkpointer and object temp relation. In addition, some I/O operations will never be performed either by certain backend types or on certain I/O objects and/or in certain I/O contexts. These cells will be NULL. For example, temporary tables are not fsynced, so fsyncs will be NULL for object temp relation. Also, the background writer does not perform reads, so reads will be NULL in rows for backend_type background writer. For the object wal, fsyncs and fsync_time track the fsync activity of WAL files done in issue_xlog_fsync. writes and write_time track the write activity of WAL files done in XLogWrite. See for more information. pg_stat_io can be used to inform database tuning. For example: A high evictions count can indicate that shared buffers should be increased. Client backends rely on the checkpointer to ensure data is persisted to permanent storage. Large numbers of fsyncs by client backends could indicate a misconfiguration of shared buffers or of the checkpointer. More information on configuring the checkpointer can be found in . Normally, client backends should be able to rely on auxiliary processes like the checkpointer and the background writer to write out dirty data as much as possible. Large numbers of writes by client backends could indicate a misconfiguration of shared buffers or of the checkpointer. More information on configuring the checkpointer can be found in . Columns tracking I/O wait time will only be non-zero when is enabled. The user should be careful when referencing these columns in combination with their corresponding I/O operations in case track_io_timing was not enabled for the entire time since the last stats reset. pg_stat_bgwriter The pg_stat_bgwriter view will always have a single row, containing data about the background writer of the cluster. Column Type Description buffers_clean bigint Number of buffers written by the background writer maxwritten_clean bigint Number of times the background writer stopped a cleaning scan because it had written too many buffers buffers_alloc bigint Number of buffers allocated stats_reset timestamp with time zone Time at which these statistics were last reset pg_stat_checkpointer The pg_stat_checkpointer view will always have a single row, containing data about the checkpointer process of the cluster. Column Type Description num_timed bigint Number of scheduled checkpoints due to timeout num_requested bigint Number of requested checkpoints num_done bigint Number of checkpoints that have been performed restartpoints_timed bigint Number of scheduled restartpoints due to timeout or after a failed attempt to perform it restartpoints_req bigint Number of requested restartpoints restartpoints_done bigint Number of restartpoints that have been performed write_time double precision Total amount of time that has been spent in the portion of processing checkpoints and restartpoints where files are written to disk, in milliseconds sync_time double precision Total amount of time that has been spent in the portion of processing checkpoints and restartpoints where files are synchronized to disk, in milliseconds buffers_written bigint Number of shared buffers written during checkpoints and restartpoints slru_written bigint Number of SLRU buffers written during checkpoints and restartpoints stats_reset timestamp with time zone Time at which these statistics were last reset Checkpoints may be skipped if the server has been idle since the last one. num_timed and num_requested count both completed and skipped checkpoints, while num_done tracks only the completed ones. Similarly, restartpoints may be skipped if the last replayed checkpoint record is already the last restartpoint. restartpoints_timed and restartpoints_req count both completed and skipped restartpoints, while restartpoints_done tracks only the completed ones. pg_stat_wal The pg_stat_wal view will always have a single row, containing data about WAL activity of the cluster. Column Type Description wal_records bigint Total number of WAL records generated wal_fpi bigint Total number of WAL full page images generated wal_bytes numeric Total amount of WAL generated in bytes wal_buffers_full bigint Number of times WAL data was written to disk because WAL buffers became full stats_reset timestamp with time zone Time at which these statistics were last reset pg_stat_database The pg_stat_database view will contain one row for each database in the cluster, plus one for shared objects, showing database-wide statistics. Column Type Description datid oid OID of this database, or 0 for objects belonging to a shared relation datname name Name of this database, or NULL for shared objects. numbackends integer Number of backends currently connected to this database, or NULL for shared objects. This is the only column in this view that returns a value reflecting current state; all other columns return the accumulated values since the last reset. xact_commit bigint Number of transactions in this database that have been committed xact_rollback bigint Number of transactions in this database that have been rolled back blks_read bigint Number of disk blocks read in this database blks_hit bigint Number of times disk blocks were found already in the buffer cache, so that a read was not necessary (this only includes hits in the PostgreSQL buffer cache, not the operating system's file system cache) tup_returned bigint Number of live rows fetched by sequential scans and index entries returned by index scans in this database tup_fetched bigint Number of live rows fetched by index scans in this database tup_inserted bigint Number of rows inserted by queries in this database tup_updated bigint Number of rows updated by queries in this database tup_deleted bigint Number of rows deleted by queries in this database conflicts bigint Number of queries canceled due to conflicts with recovery in this database. (Conflicts occur only on standby servers; see pg_stat_database_conflicts for details.) temp_files bigint Number of temporary files created by queries in this database. All temporary files are counted, regardless of why the temporary file was created (e.g., sorting or hashing), and regardless of the setting. temp_bytes bigint Total amount of data written to temporary files by queries in this database. All temporary files are counted, regardless of why the temporary file was created, and regardless of the setting. deadlocks bigint Number of deadlocks detected in this database checksum_failures bigint Number of data page checksum failures detected in this database (or on a shared object), or NULL if data checksums are disabled. checksum_last_failure timestamp with time zone Time at which the last data page checksum failure was detected in this database (or on a shared object), or NULL if data checksums are disabled. blk_read_time double precision Time spent reading data file blocks by backends in this database, in milliseconds (if is enabled, otherwise zero) blk_write_time double precision Time spent writing data file blocks by backends in this database, in milliseconds (if is enabled, otherwise zero) session_time double precision Time spent by database sessions in this database, in milliseconds (note that statistics are only updated when the state of a session changes, so if sessions have been idle for a long time, this idle time won't be included) active_time double precision Time spent executing SQL statements in this database, in milliseconds (this corresponds to the states active and fastpath function call in pg_stat_activity) idle_in_transaction_time double precision Time spent idling while in a transaction in this database, in milliseconds (this corresponds to the states idle in transaction and idle in transaction (aborted) in pg_stat_activity) sessions bigint Total number of sessions established to this database sessions_abandoned bigint Number of database sessions to this database that were terminated because connection to the client was lost sessions_fatal bigint Number of database sessions to this database that were terminated by fatal errors sessions_killed bigint Number of database sessions to this database that were terminated by operator intervention parallel_workers_to_launch bigint Number of parallel workers planned to be launched by queries on this database parallel_workers_launched bigint Number of parallel workers launched by queries on this database stats_reset timestamp with time zone Time at which these statistics were last reset pg_stat_database_conflicts The pg_stat_database_conflicts view will contain one row per database, showing database-wide statistics about query cancels occurring due to conflicts with recovery on standby servers. This view will only contain information on standby servers, since conflicts do not occur on primary servers. Column Type Description datid oid OID of a database datname name Name of this database confl_tablespace bigint Number of queries in this database that have been canceled due to dropped tablespaces confl_lock bigint Number of queries in this database that have been canceled due to lock timeouts confl_snapshot bigint Number of queries in this database that have been canceled due to old snapshots confl_bufferpin bigint Number of queries in this database that have been canceled due to pinned buffers confl_deadlock bigint Number of queries in this database that have been canceled due to deadlocks confl_active_logicalslot bigint Number of uses of logical slots in this database that have been canceled due to old snapshots or too low a on the primary pg_stat_all_tables The pg_stat_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about accesses to that specific table. The pg_stat_user_tables and pg_stat_sys_tables views contain the same information, but filtered to only show user and system tables respectively. Column Type Description relid oid OID of a table schemaname name Name of the schema that this table is in relname name Name of this table seq_scan bigint Number of sequential scans initiated on this table last_seq_scan timestamp with time zone The time of the last sequential scan on this table, based on the most recent transaction stop time seq_tup_read bigint Number of live rows fetched by sequential scans idx_scan bigint Number of index scans initiated on this table last_idx_scan timestamp with time zone The time of the last index scan on this table, based on the most recent transaction stop time idx_tup_fetch bigint Number of live rows fetched by index scans n_tup_ins bigint Total number of rows inserted n_tup_upd bigint Total number of rows updated. (This includes row updates counted in n_tup_hot_upd and n_tup_newpage_upd, and remaining non-HOT updates.) n_tup_del bigint Total number of rows deleted n_tup_hot_upd bigint Number of rows HOT updated. These are updates where no successor versions are required in indexes. n_tup_newpage_upd bigint Number of rows updated where the successor version goes onto a new heap page, leaving behind an original version with a t_ctid field that points to a different heap page. These are always non-HOT updates. n_live_tup bigint Estimated number of live rows n_dead_tup bigint Estimated number of dead rows n_mod_since_analyze bigint Estimated number of rows modified since this table was last analyzed n_ins_since_vacuum bigint Estimated number of rows inserted since this table was last vacuumed last_vacuum timestamp with time zone Last time at which this table was manually vacuumed (not counting VACUUM FULL) last_autovacuum timestamp with time zone Last time at which this table was vacuumed by the autovacuum daemon last_analyze timestamp with time zone Last time at which this table was manually analyzed last_autoanalyze timestamp with time zone Last time at which this table was analyzed by the autovacuum daemon vacuum_count bigint Number of times this table has been manually vacuumed (not counting VACUUM FULL) autovacuum_count bigint Number of times this table has been vacuumed by the autovacuum daemon analyze_count bigint Number of times this table has been manually analyzed autoanalyze_count bigint Number of times this table has been analyzed by the autovacuum daemon total_vacuum_time double precision Total time this table has been manually vacuumed, in milliseconds. (This includes the time spent sleeping due to cost-based delays.) total_autovacuum_time double precision Total time this table has been vacuumed by the autovacuum daemon, in milliseconds. (This includes the time spent sleeping due to cost-based delays.) total_analyze_time double precision Total time this table has been manually analyzed, in milliseconds. (This includes the time spent sleeping due to cost-based delays.) total_autoanalyze_time double precision Total time this table has been analyzed by the autovacuum daemon, in milliseconds. (This includes the time spent sleeping due to cost-based delays.) pg_stat_all_indexes The pg_stat_all_indexes view will contain one row for each index in the current database, showing statistics about accesses to that specific index. The pg_stat_user_indexes and pg_stat_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Column Type Description relid oid OID of the table for this index indexrelid oid OID of this index schemaname name Name of the schema this index is in relname name Name of the table for this index indexrelname name Name of this index idx_scan bigint Number of index scans initiated on this index last_idx_scan timestamp with time zone The time of the last scan on this index, based on the most recent transaction stop time idx_tup_read bigint Number of index entries returned by scans on this index idx_tup_fetch bigint Number of live table rows fetched by simple index scans using this index Indexes can be used by simple index scans, bitmap index scans, and the optimizer. In a bitmap scan the output of several indexes can be combined via AND or OR rules, so it is difficult to associate individual heap row fetches with specific indexes when a bitmap scan is used. Therefore, a bitmap scan increments the pg_stat_all_indexes.idx_tup_read count(s) for the index(es) it uses, and it increments the pg_stat_all_tables.idx_tup_fetch count for the table, but it does not affect pg_stat_all_indexes.idx_tup_fetch. The optimizer also accesses indexes to check for supplied constants whose values are outside the recorded range of the optimizer statistics because the optimizer statistics might be stale. The idx_tup_read and idx_tup_fetch counts can be different even without any use of bitmap scans, because idx_tup_read counts index entries retrieved from the index while idx_tup_fetch counts live rows fetched from the table. The latter will be less if any dead or not-yet-committed rows are fetched using the index, or if any heap fetches are avoided by means of an index-only scan. Index scans may sometimes perform multiple index searches per execution. Each index search increments pg_stat_all_indexes.idx_scan, so it's possible for the count of index scans to significantly exceed the total number of index scan executor node executions. This can happen with queries that use certain SQL constructs to search for rows matching any value out of a list or array of multiple scalar values (see ). It can also happen to queries with a column_name = value1 OR column_name = value2 ... construct, though only when the optimizer transforms the construct into an equivalent multi-valued array representation. Similarly, when B-tree index scans use the skip scan optimization, an index search is performed each time the scan is repositioned to the next index leaf page that might have matching tuples (see ). EXPLAIN ANALYZE outputs the total number of index searches performed by each index scan node. See for an example demonstrating how this works. pg_statio_all_tables The pg_statio_all_tables view will contain one row for each table in the current database (including TOAST tables), showing statistics about I/O on that specific table. The pg_statio_user_tables and pg_statio_sys_tables views contain the same information, but filtered to only show user and system tables respectively. Column Type Description relid oid OID of a table schemaname name Name of the schema that this table is in relname name Name of this table heap_blks_read bigint Number of disk blocks read from this table heap_blks_hit bigint Number of buffer hits in this table idx_blks_read bigint Number of disk blocks read from all indexes on this table idx_blks_hit bigint Number of buffer hits in all indexes on this table toast_blks_read bigint Number of disk blocks read from this table's TOAST table (if any) toast_blks_hit bigint Number of buffer hits in this table's TOAST table (if any) tidx_blks_read bigint Number of disk blocks read from this table's TOAST table indexes (if any) tidx_blks_hit bigint Number of buffer hits in this table's TOAST table indexes (if any) pg_statio_all_indexes The pg_statio_all_indexes view will contain one row for each index in the current database, showing statistics about I/O on that specific index. The pg_statio_user_indexes and pg_statio_sys_indexes views contain the same information, but filtered to only show user and system indexes respectively. Column Type Description relid oid OID of the table for this index indexrelid oid OID of this index schemaname name Name of the schema this index is in relname name Name of the table for this index indexrelname name Name of this index idx_blks_read bigint Number of disk blocks read from this index idx_blks_hit bigint Number of buffer hits in this index pg_statio_all_sequences The pg_statio_all_sequences view will contain one row for each sequence in the current database, showing statistics about I/O on that specific sequence. Column Type Description relid oid OID of a sequence schemaname name Name of the schema this sequence is in relname name Name of this sequence blks_read bigint Number of disk blocks read from this sequence blks_hit bigint Number of buffer hits in this sequence pg_stat_user_functions The pg_stat_user_functions view will contain one row for each tracked function, showing statistics about executions of that function. The parameter controls exactly which functions are tracked. Column Type Description funcid oid OID of a function schemaname name Name of the schema this function is in funcname name Name of this function calls bigint Number of times this function has been called total_time double precision Total time spent in this function and all other functions called by it, in milliseconds self_time double precision Total time spent in this function itself, not including other functions called by it, in milliseconds SLRU pg_stat_slru PostgreSQL accesses certain on-disk information via SLRU (simple least-recently-used) caches. The pg_stat_slru view will contain one row for each tracked SLRU cache, showing statistics about access to cached pages. For each SLRU cache that's part of the core server, there is a configuration parameter that controls its size, with the suffix _buffers appended. Column Type Description name text Name of the SLRU blks_zeroed bigint Number of blocks zeroed during initializations blks_hit bigint Number of times disk blocks were found already in the SLRU, so that a read was not necessary (this only includes hits in the SLRU, not the operating system's file system cache) blks_read bigint Number of disk blocks read for this SLRU blks_written bigint Number of disk blocks written for this SLRU blks_exists bigint Number of blocks checked for existence for this SLRU flushes bigint Number of flushes of dirty data for this SLRU truncates bigint Number of truncates for this SLRU stats_reset timestamp with time zone Time at which these statistics were last reset Other ways of looking at the statistics can be set up by writing queries that use the same underlying statistics access functions used by the standard views shown above. For details such as the functions' names, consult the definitions of the standard views. (For example, in psql you could issue \d+ pg_stat_activity.) The access functions for per-database statistics take a database OID as an argument to identify which database to report on. The per-table and per-index functions take a table or index OID. The functions for per-function statistics take a function OID. Note that only tables, indexes, and functions in the current database can be seen with these functions. Additional functions related to the cumulative statistics system are listed in . Function Description pg_backend_pid () integer Returns the process ID of the server process attached to the current session. pg_stat_get_backend_io pg_stat_get_backend_io ( integer ) setof record Returns I/O statistics about the backend with the specified process ID. The output fields are exactly the same as the ones in the pg_stat_io view. The function does not return I/O statistics for the checkpointer, the background writer, the startup process and the autovacuum launcher as they are already visible in the pg_stat_io view and there is only one of each. pg_stat_get_activity pg_stat_get_activity ( integer ) setof record Returns a record of information about the backend with the specified process ID, or one record for each active backend in the system if NULL is specified. The fields returned are a subset of those in the pg_stat_activity view. pg_stat_get_backend_wal pg_stat_get_backend_wal ( integer ) record Returns WAL statistics about the backend with the specified process ID. The output fields are exactly the same as the ones in the pg_stat_wal view. The function does not return WAL statistics for the checkpointer, the background writer, the startup process and the autovacuum launcher. pg_stat_get_snapshot_timestamp pg_stat_get_snapshot_timestamp () timestamp with time zone Returns the timestamp of the current statistics snapshot, or NULL if no statistics snapshot has been taken. A snapshot is taken the first time cumulative statistics are accessed in a transaction if stats_fetch_consistency is set to snapshot pg_stat_get_xact_blocks_fetched pg_stat_get_xact_blocks_fetched ( oid ) bigint Returns the number of block read requests for table or index, in the current transaction. This number minus pg_stat_get_xact_blocks_hit gives the number of kernel read() calls; the number of actual physical reads is usually lower due to kernel-level buffering. pg_stat_get_xact_blocks_hit pg_stat_get_xact_blocks_hit ( oid ) bigint Returns the number of block read requests for table or index, in the current transaction, found in cache (not triggering kernel read() calls). pg_stat_clear_snapshot pg_stat_clear_snapshot () void Discards the current statistics snapshot or cached information. pg_stat_reset pg_stat_reset () void Resets all statistics counters for the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_shared pg_stat_reset_shared ( [ target text DEFAULT NULL ] ) void Resets some cluster-wide statistics counters to zero, depending on the argument. target can be: archiver: Reset all the counters shown in the pg_stat_archiver view. bgwriter: Reset all the counters shown in the pg_stat_bgwriter view. checkpointer: Reset all the counters shown in the pg_stat_checkpointer view. io: Reset all the counters shown in the pg_stat_io view. recovery_prefetch: Reset all the counters shown in the pg_stat_recovery_prefetch view. slru: Reset all the counters shown in the pg_stat_slru view. wal: Reset all the counters shown in the pg_stat_wal view. NULL or not specified: All the counters from the views listed above are reset. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_single_table_counters pg_stat_reset_single_table_counters ( oid ) void Resets statistics for a single table or index in the current database or shared across all databases in the cluster to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_backend_stats pg_stat_reset_backend_stats ( integer ) void Resets statistics for a single backend with the specified process ID to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_single_function_counters pg_stat_reset_single_function_counters ( oid ) void Resets statistics for a single function in the current database to zero. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_slru pg_stat_reset_slru ( [ target text DEFAULT NULL ] ) void Resets statistics to zero for a single SLRU cache, or for all SLRUs in the cluster. If target is NULL or is not specified, all the counters shown in the pg_stat_slru view for all SLRU caches are reset. The argument can be one of commit_timestamp, multixact_member, multixact_offset, notify, serializable, subtransaction, or transaction to reset the counters for only that entry. If the argument is other (or indeed, any unrecognized name), then the counters for all other SLRU caches, such as extension-defined caches, are reset. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_replication_slot pg_stat_reset_replication_slot ( text ) void Resets statistics of the replication slot defined by the argument. If the argument is NULL, resets statistics for all the replication slots. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. pg_stat_reset_subscription_stats pg_stat_reset_subscription_stats ( oid ) void Resets statistics for a single subscription shown in the pg_stat_subscription_stats view to zero. If the argument is NULL, reset statistics for all subscriptions. This function is restricted to superusers by default, but other users can be granted EXECUTE to run the function. Using pg_stat_reset() also resets counters that autovacuum uses to determine when to trigger a vacuum or an analyze. Resetting these counters can cause autovacuum to not perform necessary work, which can cause problems such as table bloat or out-dated table statistics. A database-wide ANALYZE is recommended after the statistics have been reset. pg_stat_get_activity, the underlying function of the pg_stat_activity view, returns a set of records containing all the available information about each backend process. Sometimes it may be more convenient to obtain just a subset of this information. In such cases, another set of per-backend statistics access functions can be used; these are shown in . These access functions use the session's backend ID number, which is a small integer (>= 0) that is distinct from the backend ID of any concurrent session, although a session's ID can be recycled as soon as it exits. The backend ID is used, among other things, to identify the session's temporary schema if it has one. The function pg_stat_get_backend_idset provides a convenient way to list all the active backends' ID numbers for invoking these functions. For example, to show the PIDs and current queries of all backends: SELECT pg_stat_get_backend_pid(backendid) AS pid, pg_stat_get_backend_activity(backendid) AS query FROM pg_stat_get_backend_idset() AS backendid; Function Description pg_stat_get_backend_activity pg_stat_get_backend_activity ( integer ) text Returns the text of this backend's most recent query. pg_stat_get_backend_activity_start pg_stat_get_backend_activity_start ( integer ) timestamp with time zone Returns the time when the backend's most recent query was started. pg_stat_get_backend_client_addr pg_stat_get_backend_client_addr ( integer ) inet Returns the IP address of the client connected to this backend. pg_stat_get_backend_client_port pg_stat_get_backend_client_port ( integer ) integer Returns the TCP port number that the client is using for communication. pg_stat_get_backend_dbid pg_stat_get_backend_dbid ( integer ) oid Returns the OID of the database this backend is connected to. pg_stat_get_backend_idset pg_stat_get_backend_idset () setof integer Returns the set of currently active backend ID numbers. pg_stat_get_backend_pid pg_stat_get_backend_pid ( integer ) integer Returns the process ID of this backend. pg_stat_get_backend_start pg_stat_get_backend_start ( integer ) timestamp with time zone Returns the time when this process was started. pg_stat_get_backend_subxact pg_stat_get_backend_subxact ( integer ) record Returns a record of information about the subtransactions of the backend with the specified ID. The fields returned are subxact_count, which is the number of subtransactions in the backend's subtransaction cache, and subxact_overflow, which indicates whether the backend's subtransaction cache is overflowed or not. pg_stat_get_backend_userid pg_stat_get_backend_userid ( integer ) oid Returns the OID of the user logged into this backend. pg_stat_get_backend_wait_event pg_stat_get_backend_wait_event ( integer ) text Returns the wait event name if this backend is currently waiting, otherwise NULL. See through . pg_stat_get_backend_wait_event_type pg_stat_get_backend_wait_event_type ( integer ) text Returns the wait event type name if this backend is currently waiting, otherwise NULL. See for details. pg_stat_get_backend_xact_start pg_stat_get_backend_xact_start ( integer ) timestamp with time zone Returns the time when the backend's current transaction was started. lock monitoring Another useful tool for monitoring database activity is the pg_locks system table. It allows the database administrator to view information about the outstanding locks in the lock manager. For example, this capability can be used to: View all the locks currently outstanding, all the locks on relations in a particular database, all the locks on a particular relation, or all the locks held by a particular PostgreSQL session. Determine the relation in the current database with the most ungranted locks (which might be a source of contention among database clients). Determine the effect of lock contention on overall database performance, as well as the extent to which contention varies with overall database traffic. Details of the pg_locks view appear in . For more information on locking and managing concurrency with PostgreSQL, refer to . PostgreSQL has the ability to report the progress of certain commands during command execution. Currently, the only commands which support progress reporting are ANALYZE, CLUSTER, CREATE INDEX, VACUUM, COPY, and (i.e., replication command that issues to take a base backup). This may be expanded in the future. pg_stat_progress_analyze Whenever ANALYZE is running, the pg_stat_progress_analyze view will contain a row for each backend that is currently running that command. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table being analyzed. phase text Current processing phase. See . sample_blks_total bigint Total number of heap blocks that will be sampled. sample_blks_scanned bigint Number of heap blocks scanned. ext_stats_total bigint Number of extended statistics. ext_stats_computed bigint Number of extended statistics computed. This counter only advances when the phase is computing extended statistics. child_tables_total bigint Number of child tables. child_tables_done bigint Number of child tables scanned. This counter only advances when the phase is acquiring inherited sample rows. current_child_table_relid oid OID of the child table currently being scanned. This field is only valid when the phase is acquiring inherited sample rows. delay_time double precision Total time spent sleeping due to cost-based delay (see , in milliseconds (if is enabled, otherwise zero). Phase Description initializing The command is preparing to begin scanning the heap. This phase is expected to be very brief. acquiring sample rows The command is currently scanning the table given by relid to obtain sample rows. acquiring inherited sample rows The command is currently scanning child tables to obtain sample rows. Columns child_tables_total, child_tables_done, and current_child_table_relid contain the progress information for this phase. computing statistics The command is computing statistics from the sample rows obtained during the table scan. computing extended statistics The command is computing extended statistics from the sample rows obtained during the table scan. finalizing analyze The command is updating pg_class. When this phase is completed, ANALYZE will end. Note that when ANALYZE is run on a partitioned table without the ONLY keyword, all of its partitions are also recursively analyzed. In that case, ANALYZE progress is reported first for the parent table, whereby its inheritance statistics are collected, followed by that for each partition. pg_stat_progress_cluster Whenever CLUSTER or VACUUM FULL is running, the pg_stat_progress_cluster view will contain a row for each backend that is currently running either command. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table being clustered. command text The command that is running. Either CLUSTER or VACUUM FULL. phase text Current processing phase. See . cluster_index_relid oid If the table is being scanned using an index, this is the OID of the index being used; otherwise, it is zero. heap_tuples_scanned bigint Number of heap tuples scanned. This counter only advances when the phase is seq scanning heap, index scanning heap or writing new heap. heap_tuples_written bigint Number of heap tuples written. This counter only advances when the phase is seq scanning heap, index scanning heap or writing new heap. heap_blks_total bigint Total number of heap blocks in the table. This number is reported as of the beginning of seq scanning heap. heap_blks_scanned bigint Number of heap blocks scanned. This counter only advances when the phase is seq scanning heap. index_rebuild_count bigint Number of indexes rebuilt. This counter only advances when the phase is rebuilding index. Phase Description initializing The command is preparing to begin scanning the heap. This phase is expected to be very brief. seq scanning heap The command is currently scanning the table using a sequential scan. index scanning heap CLUSTER is currently scanning the table using an index scan. sorting tuples CLUSTER is currently sorting tuples. writing new heap CLUSTER is currently writing the new heap. swapping relation files The command is currently swapping newly-built files into place. rebuilding index The command is currently rebuilding an index. performing final cleanup The command is performing final cleanup. When this phase is completed, CLUSTER or VACUUM FULL will end. pg_stat_progress_copy Whenever COPY is running, the pg_stat_progress_copy view will contain one row for each backend that is currently running a COPY command. The table below describes the information that will be reported and provides information about how to interpret it. Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table on which the COPY command is executed. It is set to 0 if copying from a SELECT query. command text The command that is running: COPY FROM, or COPY TO. type text The I/O type that the data is read from or written to: FILE, PROGRAM, PIPE (for COPY FROM STDIN and COPY TO STDOUT), or CALLBACK (used for example during the initial table synchronization in logical replication). bytes_processed bigint Number of bytes already processed by COPY command. bytes_total bigint Size of source file for COPY FROM command in bytes. It is set to 0 if not available. tuples_processed bigint Number of tuples already processed by COPY command. tuples_excluded bigint Number of tuples not processed because they were excluded by the WHERE clause of the COPY command. tuples_skipped bigint Number of tuples skipped because they contain malformed data. This counter only advances when a value other than stop is specified to the ON_ERROR option. pg_stat_progress_create_index Whenever CREATE INDEX or REINDEX is running, the pg_stat_progress_create_index view will contain one row for each backend that is currently creating indexes. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of the backend creating indexes. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table on which the index is being created. index_relid oid OID of the index being created or reindexed. During a non-concurrent CREATE INDEX, this is 0. command text Specific command type: CREATE INDEX, CREATE INDEX CONCURRENTLY, REINDEX, or REINDEX CONCURRENTLY. phase text Current processing phase of index creation. See . lockers_total bigint Total number of lockers to wait for, when applicable. lockers_done bigint Number of lockers already waited for. current_locker_pid bigint Process ID of the locker currently being waited for. blocks_total bigint Total number of blocks to be processed in the current phase. blocks_done bigint Number of blocks already processed in the current phase. tuples_total bigint Total number of tuples to be processed in the current phase. tuples_done bigint Number of tuples already processed in the current phase. partitions_total bigint Total number of partitions on which the index is to be created or attached, including both direct and indirect partitions. 0 during a REINDEX, or when the index is not partitioned. partitions_done bigint Number of partitions on which the index has already been created or attached, including both direct and indirect partitions. 0 during a REINDEX, or when the index is not partitioned. Phase Description initializing CREATE INDEX or REINDEX is preparing to create the index. This phase is expected to be very brief. waiting for writers before build CREATE INDEX CONCURRENTLY or REINDEX CONCURRENTLY is waiting for transactions with write locks that can potentially see the table to finish. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. building index The index is being built by the access method-specific code. In this phase, access methods that support progress reporting fill in their own progress data, and the subphase is indicated in this column. Typically, blocks_total and blocks_done will contain progress data, as well as potentially tuples_total and tuples_done. waiting for writers before validation CREATE INDEX CONCURRENTLY or REINDEX CONCURRENTLY is waiting for transactions with write locks that can potentially write into the table to finish. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. index validation: scanning index CREATE INDEX CONCURRENTLY is scanning the index searching for tuples that need to be validated. This phase is skipped when not in concurrent mode. Columns blocks_total (set to the total size of the index) and blocks_done contain the progress information for this phase. index validation: sorting tuples CREATE INDEX CONCURRENTLY is sorting the output of the index scanning phase. index validation: scanning table CREATE INDEX CONCURRENTLY is scanning the table to validate the index tuples collected in the previous two phases. This phase is skipped when not in concurrent mode. Columns blocks_total (set to the total size of the table) and blocks_done contain the progress information for this phase. waiting for old snapshots CREATE INDEX CONCURRENTLY or REINDEX CONCURRENTLY is waiting for transactions that can potentially see the table to release their snapshots. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. waiting for readers before marking dead REINDEX CONCURRENTLY is waiting for transactions with read locks on the table to finish, before marking the old index dead. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. waiting for readers before dropping REINDEX CONCURRENTLY is waiting for transactions with read locks on the table to finish, before dropping the old index. This phase is skipped when not in concurrent mode. Columns lockers_total, lockers_done and current_locker_pid contain the progress information for this phase. pg_stat_progress_vacuum Whenever VACUUM is running, the pg_stat_progress_vacuum view will contain one row for each backend (including autovacuum worker processes) that is currently vacuuming. The tables below describe the information that will be reported and provide information about how to interpret it. Progress for VACUUM FULL commands is reported via pg_stat_progress_cluster because both VACUUM FULL and CLUSTER rewrite the table, while regular VACUUM only modifies it in place. See . Column Type Description pid integer Process ID of backend. datid oid OID of the database to which this backend is connected. datname name Name of the database to which this backend is connected. relid oid OID of the table being vacuumed. phase text Current processing phase of vacuum. See . heap_blks_total bigint Total number of heap blocks in the table. This number is reported as of the beginning of the scan; blocks added later will not be (and need not be) visited by this VACUUM. heap_blks_scanned bigint Number of heap blocks scanned. Because the visibility map is used to optimize scans, some blocks will be skipped without inspection; skipped blocks are included in this total, so that this number will eventually become equal to heap_blks_total when the vacuum is complete. This counter only advances when the phase is scanning heap. heap_blks_vacuumed bigint Number of heap blocks vacuumed. Unless the table has no indexes, this counter only advances when the phase is vacuuming heap. Blocks that contain no dead tuples are skipped, so the counter may sometimes skip forward in large increments. index_vacuum_count bigint Number of completed index vacuum cycles. max_dead_tuple_bytes bigint Amount of dead tuple data that we can store before needing to perform an index vacuum cycle, based on . dead_tuple_bytes bigint Amount of dead tuple data collected since the last index vacuum cycle. num_dead_item_ids bigint Number of dead item identifiers collected since the last index vacuum cycle. indexes_total bigint Total number of indexes that will be vacuumed or cleaned up. This number is reported at the beginning of the vacuuming indexes phase or the cleaning up indexes phase. indexes_processed bigint Number of indexes processed. This counter only advances when the phase is vacuuming indexes or cleaning up indexes. delay_time double precision Total time spent sleeping due to cost-based delay (see ), in milliseconds (if is enabled, otherwise zero). This includes the time that any associated parallel workers have slept. However, parallel workers report their sleep time no more frequently than once per second, so the reported value may be slightly stale. Phase Description initializing VACUUM is preparing to begin scanning the heap. This phase is expected to be very brief. scanning heap VACUUM is currently scanning the heap. It will prune and defragment each page if required, and possibly perform freezing activity. The heap_blks_scanned column can be used to monitor the progress of the scan. vacuuming indexes VACUUM is currently vacuuming the indexes. If a table has any indexes, this will happen at least once per vacuum, after the heap has been completely scanned. It may happen multiple times per vacuum if (or, in the case of autovacuum, if set) is insufficient to store the number of dead tuples found. vacuuming heap VACUUM is currently vacuuming the heap. Vacuuming the heap is distinct from scanning the heap, and occurs after each instance of vacuuming indexes. If heap_blks_scanned is less than heap_blks_total, the system will return to scanning the heap after this phase is completed; otherwise, it will begin cleaning up indexes after this phase is completed. cleaning up indexes VACUUM is currently cleaning up indexes. This occurs after the heap has been completely scanned and all vacuuming of the indexes and the heap has been completed. truncating heap VACUUM is currently truncating the heap so as to return empty pages at the end of the relation to the operating system. This occurs after cleaning up indexes. performing final cleanup VACUUM is performing final cleanup. During this phase, VACUUM will vacuum the free space map, update statistics in pg_class, and report statistics to the cumulative statistics system. When this phase is completed, VACUUM will end. pg_stat_progress_basebackup Whenever an application like pg_basebackup is taking a base backup, the pg_stat_progress_basebackup view will contain a row for each WAL sender process that is currently running the BASE_BACKUP replication command and streaming the backup. The tables below describe the information that will be reported and provide information about how to interpret it. Column Type Description pid integer Process ID of a WAL sender process. phase text Current processing phase. See . backup_total bigint Total amount of data that will be streamed. This is estimated and reported as of the beginning of streaming database files phase. Note that this is only an approximation since the database may change during streaming database files phase and WAL log may be included in the backup later. This is always the same value as backup_streamed once the amount of data streamed exceeds the estimated total size. If the estimation is disabled in pg_basebackup (i.e., --no-estimate-size option is specified), this is NULL. backup_streamed bigint Amount of data streamed. This counter only advances when the phase is streaming database files or transferring wal files. tablespaces_total bigint Total number of tablespaces that will be streamed. tablespaces_streamed bigint Number of tablespaces streamed. This counter only advances when the phase is streaming database files. Phase Description initializing The WAL sender process is preparing to begin the backup. This phase is expected to be very brief. waiting for checkpoint to finish The WAL sender process is currently performing pg_backup_start to prepare to take a base backup, and waiting for the start-of-backup checkpoint to finish. estimating backup size The WAL sender process is currently estimating the total amount of database files that will be streamed as a base backup. streaming database files The WAL sender process is currently streaming database files as a base backup. waiting for wal archiving to finish The WAL sender process is currently performing pg_backup_stop to finish the backup, and waiting for all the WAL files required for the base backup to be successfully archived. If either --wal-method=none or --wal-method=stream is specified in pg_basebackup, the backup will end when this phase is completed. transferring wal files The WAL sender process is currently transferring all WAL logs generated during the backup. This phase occurs after waiting for wal archiving to finish phase if --wal-method=fetch is specified in pg_basebackup. The backup will end when this phase is completed. DTrace PostgreSQL provides facilities to support dynamic tracing of the database server. This allows an external utility to be called at specific points in the code and thereby trace execution. A number of probes or trace points are already inserted into the source code. These probes are intended to be used by database developers and administrators. By default the probes are not compiled into PostgreSQL; the user needs to explicitly tell the configure script to make the probes available. Currently, the DTrace utility is supported, which, at the time of this writing, is available on Solaris, macOS, FreeBSD, NetBSD, and Oracle Linux. The SystemTap project for Linux provides a DTrace equivalent and can also be used. Supporting other dynamic tracing utilities is theoretically possible by changing the definitions for the macros in src/include/utils/probes.h. By default, probes are not available, so you will need to explicitly tell the configure script to make the probes available in PostgreSQL. To include DTrace support specify --enable-dtrace to configure. See for further information. A number of standard probes are provided in the source code, as shown in ; shows the types used in the probes. More probes can certainly be added to enhance PostgreSQL's observability. Name Parameters Description transaction-start (LocalTransactionId) Probe that fires at the start of a new transaction. arg0 is the transaction ID. transaction-commit (LocalTransactionId) Probe that fires when a transaction completes successfully. arg0 is the transaction ID. transaction-abort (LocalTransactionId) Probe that fires when a transaction completes unsuccessfully. arg0 is the transaction ID. query-start (const char *) Probe that fires when the processing of a query is started. arg0 is the query string. query-done (const char *) Probe that fires when the processing of a query is complete. arg0 is the query string. query-parse-start (const char *) Probe that fires when the parsing of a query is started. arg0 is the query string. query-parse-done (const char *) Probe that fires when the parsing of a query is complete. arg0 is the query string. query-rewrite-start (const char *) Probe that fires when the rewriting of a query is started. arg0 is the query string. query-rewrite-done (const char *) Probe that fires when the rewriting of a query is complete. arg0 is the query string. query-plan-start () Probe that fires when the planning of a query is started. query-plan-done () Probe that fires when the planning of a query is complete. query-execute-start () Probe that fires when the execution of a query is started. query-execute-done () Probe that fires when the execution of a query is complete. statement-status (const char *) Probe that fires anytime the server process updates its pg_stat_activity.status. arg0 is the new status string. checkpoint-start (int) Probe that fires when a checkpoint is started. arg0 holds the bitwise flags used to distinguish different checkpoint types, such as shutdown, immediate or force. checkpoint-done (int, int, int, int, int) Probe that fires when a checkpoint is complete. (The probes listed next fire in sequence during checkpoint processing.) arg0 is the number of buffers written. arg1 is the total number of buffers. arg2, arg3 and arg4 contain the number of WAL files added, removed and recycled respectively. clog-checkpoint-start (bool) Probe that fires when the CLOG portion of a checkpoint is started. arg0 is true for normal checkpoint, false for shutdown checkpoint. clog-checkpoint-done (bool) Probe that fires when the CLOG portion of a checkpoint is complete. arg0 has the same meaning as for clog-checkpoint-start. subtrans-checkpoint-start (bool) Probe that fires when the SUBTRANS portion of a checkpoint is started. arg0 is true for normal checkpoint, false for shutdown checkpoint. subtrans-checkpoint-done (bool) Probe that fires when the SUBTRANS portion of a checkpoint is complete. arg0 has the same meaning as for subtrans-checkpoint-start. multixact-checkpoint-start (bool) Probe that fires when the MultiXact portion of a checkpoint is started. arg0 is true for normal checkpoint, false for shutdown checkpoint. multixact-checkpoint-done (bool) Probe that fires when the MultiXact portion of a checkpoint is complete. arg0 has the same meaning as for multixact-checkpoint-start. buffer-checkpoint-start (int) Probe that fires when the buffer-writing portion of a checkpoint is started. arg0 holds the bitwise flags used to distinguish different checkpoint types, such as shutdown, immediate or force. buffer-sync-start (int, int) Probe that fires when we begin to write dirty buffers during checkpoint (after identifying which buffers must be written). arg0 is the total number of buffers. arg1 is the number that are currently dirty and need to be written. buffer-sync-written (int) Probe that fires after each buffer is written during checkpoint. arg0 is the ID number of the buffer. buffer-sync-done (int, int, int) Probe that fires when all dirty buffers have been written. arg0 is the total number of buffers. arg1 is the number of buffers actually written by the checkpoint process. arg2 is the number that were expected to be written (arg1 of buffer-sync-start); any difference reflects other processes flushing buffers during the checkpoint. buffer-checkpoint-sync-start () Probe that fires after dirty buffers have been written to the kernel, and before starting to issue fsync requests. buffer-checkpoint-done () Probe that fires when syncing of buffers to disk is complete. twophase-checkpoint-start () Probe that fires when the two-phase portion of a checkpoint is started. twophase-checkpoint-done () Probe that fires when the two-phase portion of a checkpoint is complete. buffer-extend-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int, unsigned int) Probe that fires when a relation extension starts. arg0 contains the fork to be extended. arg1, arg2, and arg3 contain the tablespace, database, and relation OIDs identifying the relation. arg4 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. arg5 is the number of blocks the caller would like to extend by. buffer-extend-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, unsigned int, BlockNumber) Probe that fires when a relation extension is complete. arg0 contains the fork to be extended. arg1, arg2, and arg3 contain the tablespace, database, and relation OIDs identifying the relation. arg4 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. arg5 is the number of blocks the relation was extended by, this can be less than the number in the buffer-extend-start due to resource constraints. arg6 contains the BlockNumber of the first new block. buffer-read-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int) Probe that fires when a buffer read is started. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. buffer-read-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, bool) Probe that fires when a buffer read is complete. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. arg6 is true if the buffer was found in the pool, false if not. buffer-flush-start (ForkNumber, BlockNumber, Oid, Oid, Oid) Probe that fires before issuing any write request for a shared buffer. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. buffer-flush-done (ForkNumber, BlockNumber, Oid, Oid, Oid) Probe that fires when a write request is complete. (Note that this just reflects the time to pass the data to the kernel; it's typically not actually been written to disk yet.) The arguments are the same as for buffer-flush-start. wal-buffer-write-dirty-start () Probe that fires when a server process begins to write a dirty WAL buffer because no more WAL buffer space is available. (If this happens often, it implies that is too small.) wal-buffer-write-dirty-done () Probe that fires when a dirty WAL buffer write is complete. wal-insert (unsigned char, unsigned char) Probe that fires when a WAL record is inserted. arg0 is the resource manager (rmid) for the record. arg1 contains the info flags. wal-switch () Probe that fires when a WAL segment switch is requested. smgr-md-read-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int) Probe that fires when beginning to read a block from a relation. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. smgr-md-read-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int) Probe that fires when a block read is complete. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. arg6 is the number of bytes actually read, while arg7 is the number requested (if these are different it indicates a short read). smgr-md-write-start (ForkNumber, BlockNumber, Oid, Oid, Oid, int) Probe that fires when beginning to write a block to a relation. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. smgr-md-write-done (ForkNumber, BlockNumber, Oid, Oid, Oid, int, int, int) Probe that fires when a block write is complete. arg0 and arg1 contain the fork and block numbers of the page. arg2, arg3, and arg4 contain the tablespace, database, and relation OIDs identifying the relation. arg5 is the ID of the backend which created the temporary relation for a local buffer, or INVALID_PROC_NUMBER (-1) for a shared buffer. arg6 is the number of bytes actually written, while arg7 is the number requested (if these are different it indicates a short write). sort-start (int, bool, int, int, bool, int) Probe that fires when a sort operation is started. arg0 indicates heap, index or datum sort. arg1 is true for unique-value enforcement. arg2 is the number of key columns. arg3 is the number of kilobytes of work memory allowed. arg4 is true if random access to the sort result is required. arg5 indicates serial when 0, parallel worker when 1, or parallel leader when 2. sort-done (bool, long) Probe that fires when a sort is complete. arg0 is true for external sort, false for internal sort. arg1 is the number of disk blocks used for an external sort, or kilobytes of memory used for an internal sort. lwlock-acquire (char *, LWLockMode) Probe that fires when an LWLock has been acquired. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-release (char *) Probe that fires when an LWLock has been released (but note that any released waiters have not yet been awakened). arg0 is the LWLock's tranche. lwlock-wait-start (char *, LWLockMode) Probe that fires when an LWLock was not immediately available and a server process has begun to wait for the lock to become available. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-wait-done (char *, LWLockMode) Probe that fires when a server process has been released from its wait for an LWLock (it does not actually have the lock yet). arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-condacquire (char *, LWLockMode) Probe that fires when an LWLock was successfully acquired when the caller specified no waiting. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lwlock-condacquire-fail (char *, LWLockMode) Probe that fires when an LWLock was not successfully acquired when the caller specified no waiting. arg0 is the LWLock's tranche. arg1 is the requested lock mode, either exclusive or shared. lock-wait-start (unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE) Probe that fires when a request for a heavyweight lock (lmgr lock) has begun to wait because the lock is not available. arg0 through arg3 are the tag fields identifying the object being locked. arg4 indicates the type of object being locked. arg5 indicates the lock type being requested. lock-wait-done (unsigned int, unsigned int, unsigned int, unsigned int, unsigned int, LOCKMODE) Probe that fires when a request for a heavyweight lock (lmgr lock) has finished waiting (i.e., has acquired the lock). The arguments are the same as for lock-wait-start. deadlock-found () Probe that fires when a deadlock is found by the deadlock detector. Type Definition LocalTransactionId unsigned int LWLockMode int LOCKMODE int BlockNumber unsigned int Oid unsigned int ForkNumber int bool unsigned char The example below shows a DTrace script for analyzing transaction counts in the system, as an alternative to snapshotting pg_stat_database before and after a performance test: #!/usr/sbin/dtrace -qs postgresql$1:::transaction-start { @start["Start"] = count(); self->ts = timestamp; } postgresql$1:::transaction-abort { @abort["Abort"] = count(); } postgresql$1:::transaction-commit /self->ts/ { @commit["Commit"] = count(); @time["Total time (ns)"] = sum(timestamp - self->ts); self->ts=0; } When executed, the example D script gives output such as: # ./txn_count.d `pgrep -n postgres` or ./txn_count.d <PID> ^C Start 71 Commit 70 Total time (ns) 2312105013 SystemTap uses a different notation for trace scripts than DTrace does, even though the underlying trace points are compatible. One point worth noting is that at this writing, SystemTap scripts must reference probe names using double underscores in place of hyphens. This is expected to be fixed in future SystemTap releases. You should remember that DTrace scripts need to be carefully written and debugged, otherwise the trace information collected might be meaningless. In most cases where problems are found it is the instrumentation that is at fault, not the underlying system. When discussing information found using dynamic tracing, be sure to enclose the script used to allow that too to be checked and discussed. New probes can be defined within the code wherever the developer desires, though this will require a recompilation. Below are the steps for inserting new probes: Decide on probe names and data to be made available through the probes Add the probe definitions to src/backend/utils/probes.d Include pg_trace.h if it is not already present in the module(s) containing the probe points, and insert TRACE_POSTGRESQL probe macros at the desired locations in the source code Recompile and verify that the new probes are available Here is an example of how you would add a probe to trace all new transactions by transaction ID. Decide that the probe will be named transaction-start and requires a parameter of type LocalTransactionId Add the probe definition to src/backend/utils/probes.d: probe transaction__start(LocalTransactionId); Note the use of the double underline in the probe name. In a DTrace script using the probe, the double underline needs to be replaced with a hyphen, so transaction-start is the name to document for users. At compile time, transaction__start is converted to a macro called TRACE_POSTGRESQL_TRANSACTION_START (notice the underscores are single here), which is available by including pg_trace.h. Add the macro call to the appropriate location in the source code. In this case, it looks like the following: TRACE_POSTGRESQL_TRANSACTION_START(vxid.localTransactionId); After recompiling and running the new binary, check that your newly added probe is available by executing the following DTrace command. You should see similar output: # dtrace -ln transaction-start ID PROVIDER MODULE FUNCTION NAME 18705 postgresql49878 postgres StartTransactionCommand transaction-start 18755 postgresql49877 postgres StartTransactionCommand transaction-start 18805 postgresql49876 postgres StartTransactionCommand transaction-start 18855 postgresql49875 postgres StartTransactionCommand transaction-start 18986 postgresql49873 postgres StartTransactionCommand transaction-start There are a few things to be careful about when adding trace macros to the C code: You should take care that the data types specified for a probe's parameters match the data types of the variables used in the macro. Otherwise, you will get compilation errors. On most platforms, if PostgreSQL is built with --enable-dtrace, the arguments to a trace macro will be evaluated whenever control passes through the macro, even if no tracing is being done. This is usually not worth worrying about if you are just reporting the values of a few local variables. But beware of putting expensive function calls into the arguments. If you need to do that, consider protecting the macro with a check to see if the trace is actually enabled: if (TRACE_POSTGRESQL_TRANSACTION_START_ENABLED()) TRACE_POSTGRESQL_TRANSACTION_START(some_function(...)); Each trace macro has a corresponding ENABLED macro. This section discusses how to monitor the disk usage of a PostgreSQL database system. disk usage Each table has a primary heap disk file where most of the data is stored. If the table has any columns with potentially-wide values, there also might be a TOAST file associated with the table, which is used to store values too wide to fit comfortably in the main table (see ). There will be one valid index on the TOAST table, if present. There also might be indexes associated with the base table. Each table and index is stored in a separate disk file — possibly more than one file, if the file would exceed one gigabyte. Naming conventions for these files are described in . You can monitor disk space in three ways: using the SQL functions listed in , using the module, or using manual inspection of the system catalogs. The SQL functions are the easiest to use and are generally recommended. The remainder of this section shows how to do it by inspection of the system catalogs. Using psql on a recently vacuumed or analyzed database, you can issue queries to see the disk usage of any table: SELECT pg_relation_filepath(oid), relpages FROM pg_class WHERE relname = 'customer'; pg_relation_filepath | relpages ----------------------+---------- base/16384/16806 | 60 (1 row) Each page is typically 8 kilobytes. (Remember, relpages is only updated by VACUUM, ANALYZE, and a few DDL commands such as CREATE INDEX.) The file path name is of interest if you want to examine the table's disk file directly. To show the space used by TOAST tables, use a query like the following: SELECT relname, relpages FROM pg_class, (SELECT reltoastrelid FROM pg_class WHERE relname = 'customer') AS ss WHERE oid = ss.reltoastrelid OR oid = (SELECT indexrelid FROM pg_index WHERE indrelid = ss.reltoastrelid) ORDER BY relname; relname | relpages ----------------------+---------- pg_toast_16806 | 0 pg_toast_16806_index | 1 You can easily display index sizes, too: SELECT c2.relname, c2.relpages FROM pg_class c, pg_class c2, pg_index i WHERE c.relname = 'customer' AND c.oid = i.indrelid AND c2.oid = i.indexrelid ORDER BY c2.relname; relname | relpages -------------------+---------- customer_id_index | 26 It is easy to find your largest tables and indexes using this information: SELECT relname, relpages FROM pg_class ORDER BY relpages DESC; relname | relpages ----------------------+---------- bigtable | 3290 customer | 3144 The most important disk monitoring task of a database administrator is to make sure the disk doesn't become full. A filled data disk will not result in data corruption, but it might prevent useful activity from occurring. If the disk holding the WAL files grows full, database server panic and consequent shutdown might occur. If you cannot free up additional space on the disk by deleting other things, you can move some of the database files to other file systems by making use of tablespaces. See for more information about that. Some file systems perform badly when they are almost full, so do not wait until the disk is completely full to take action. If your system supports per-user disk quotas, then the database will naturally be subject to whatever quota is placed on the user the server runs as. Exceeding the quota will have the same bad effects as running out of disk space entirely.