pg_buffercache The pg_buffercache module provides a means for examining what's happening in the shared buffer cache in real time. It also offers a low-level way to evict data from it, for testing purposes. pg_buffercache_pages pg_buffercache_summary pg_buffercache_evict pg_buffercache_evict_relation pg_buffercache_evict_all This module provides the pg_buffercache_pages() function (wrapped in the pg_buffercache view), pg_buffercache_numa_pages() function (wrapped in the pg_buffercache_numa view), the pg_buffercache_summary() function, the pg_buffercache_usage_counts() function, the pg_buffercache_evict(), the pg_buffercache_evict_relation() function and the pg_buffercache_evict_all() function. The pg_buffercache_pages() function returns a set of records, each row describing the state of one shared buffer entry. The pg_buffercache view wraps the function for convenient use. The pg_buffercache_numa_pages() provides NUMA node mappings for shared buffer entries. This information is not part of pg_buffercache_pages() itself, as it is much slower to retrieve. The pg_buffercache_numa view wraps the function for convenient use. The pg_buffercache_summary() function returns a single row summarizing the state of the shared buffer cache. The pg_buffercache_usage_counts() function returns a set of records, each row describing the number of buffers with a given usage count. By default, use of the above functions is restricted to superusers and roles with privileges of the pg_monitor role. Access may be granted to others using GRANT. The pg_buffercache_evict() function allows a block to be evicted from the buffer pool given a buffer identifier. Use of this function is restricted to superusers only. The pg_buffercache_evict_relation() function allows all unpinned shared buffers in the relation to be evicted from the buffer pool given a relation identifier. Use of this function is restricted to superusers only. The pg_buffercache_evict_all() function allows all unpinned shared buffers to be evicted in the buffer pool. Use of this function is restricted to superusers only. The definitions of the columns exposed by the view are shown in . Column Type Description bufferid integer ID, in the range 1..shared_buffers relfilenode oid (references pg_class.relfilenode) Filenode number of the relation reltablespace oid (references pg_tablespace.oid) Tablespace OID of the relation reldatabase oid (references pg_database.oid) Database OID of the relation relforknumber smallint Fork number within the relation; see common/relpath.h relblocknumber bigint Page number within the relation isdirty boolean Is the page dirty? usagecount smallint Clock-sweep access count pinning_backends integer Number of backends pinning this buffer There is one row for each buffer in the shared cache. Unused buffers are shown with all fields null except bufferid. Shared system catalogs are shown as belonging to database zero. Because the cache is shared by all the databases, there will normally be pages from relations not belonging to the current database. This means that there may not be matching join rows in pg_class for some rows, or that there could even be incorrect joins. If you are trying to join against pg_class, it's a good idea to restrict the join to rows having reldatabase equal to the current database's OID or zero. Since buffer manager locks are not taken to copy the buffer state data that the view will display, accessing pg_buffercache view has less impact on normal buffer activity but it doesn't provide a consistent set of results across all buffers. However, we ensure that the information of each buffer is self-consistent. The definitions of the columns exposed by the view are shown in . Column Type Description bufferid integer ID, in the range 1..shared_buffers os_page_num bigint number of OS memory page for this buffer numa_node int ID of NUMA node As NUMA node ID inquiry for each page requires memory pages to be paged-in, the first execution of this function can take a noticeable amount of time. In all the cases (first execution or not), retrieving this information is costly and querying the view at a high frequency is not recommended. When determining the NUMA node, the view touches all memory pages for the shared memory segment. This will force allocation of the shared memory, if it wasn't allocated already, and the memory may get allocated in a single NUMA node (depending on system configuration). The definitions of the columns exposed by the function are shown in . Column Type Description buffers_used int4 Number of used shared buffers buffers_unused int4 Number of unused shared buffers buffers_dirty int4 Number of dirty shared buffers buffers_pinned int4 Number of pinned shared buffers usagecount_avg float8 Average usage count of used shared buffers The pg_buffercache_summary() function returns a single row summarizing the state of all shared buffers. Similar and more detailed information is provided by the pg_buffercache view, but pg_buffercache_summary() is significantly cheaper. Like the pg_buffercache view, pg_buffercache_summary() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result. The definitions of the columns exposed by the function are shown in . Column Type Description usage_count int4 A possible buffer usage count buffers int4 Number of buffers with the usage count dirty int4 Number of dirty buffers with the usage count pinned int4 Number of pinned buffers with the usage count The pg_buffercache_usage_counts() function returns a set of rows summarizing the states of all shared buffers, aggregated over the possible usage count values. Similar and more detailed information is provided by the pg_buffercache view, but pg_buffercache_usage_counts() is significantly cheaper. Like the pg_buffercache view, pg_buffercache_usage_counts() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result. The pg_buffercache_evict() function takes a buffer identifier, as shown in the bufferid column of the pg_buffercache view. It returns information about whether the buffer was evicted and flushed. The buffer_evicted column is true on success, and false if the buffer wasn't valid, if it couldn't be evicted because it was pinned, or if it became dirty again after an attempt to write it out. The buffer_flushed column is true if the buffer was flushed. This does not necessarily mean that buffer was flushed by us, it might be flushed by someone else. The result is immediately out of date upon return, as the buffer might become valid again at any time due to concurrent activity. The function is intended for developer testing only. The pg_buffercache_evict_relation() function is very similar to the pg_buffercache_evict() function. The difference is that the pg_buffercache_evict_relation() takes a relation identifier instead of buffer identifier. It tries to evict all buffers for all forks in that relation. It returns the number of evicted buffers, flushed buffers and the number of buffers that could not be evicted. Flushed buffers haven't necessarily been flushed by us, they might have been flushed by someone else. The result is immediately out of date upon return, as buffers might immediately be read back in due to concurrent activity. The function is intended for developer testing only. The pg_buffercache_evict_all() function is very similar to the pg_buffercache_evict() function. The difference is, the pg_buffercache_evict_all() function does not take an argument; instead it tries to evict all buffers in the buffer pool. It returns the number of evicted buffers, flushed buffers and the number of buffers that could not be evicted. Flushed buffers haven't necessarily been flushed by us, they might have been flushed by someone else. The result is immediately out of date upon return, as buffers might immediately be read back in due to concurrent activity. The function is intended for developer testing only. regression=# SELECT n.nspname, c.relname, count(*) AS buffers FROM pg_buffercache b JOIN pg_class c ON b.relfilenode = pg_relation_filenode(c.oid) AND b.reldatabase IN (0, (SELECT oid FROM pg_database WHERE datname = current_database())) JOIN pg_namespace n ON n.oid = c.relnamespace GROUP BY n.nspname, c.relname ORDER BY 3 DESC LIMIT 10; nspname | relname | buffers ------------+------------------------+--------- public | delete_test_table | 593 public | delete_test_table_pkey | 494 pg_catalog | pg_attribute | 472 public | quad_poly_tbl | 353 public | tenk2 | 349 public | tenk1 | 349 public | gin_test_idx | 306 pg_catalog | pg_largeobject | 206 public | gin_test_tbl | 188 public | spgist_text_tbl | 182 (10 rows) regression=# SELECT * FROM pg_buffercache_summary(); buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg --------------+----------------+---------------+----------------+---------------- 248 | 2096904 | 39 | 0 | 3.141129 (1 row) regression=# SELECT * FROM pg_buffercache_usage_counts(); usage_count | buffers | dirty | pinned -------------+---------+-------+-------- 0 | 14650 | 0 | 0 1 | 1436 | 671 | 0 2 | 102 | 88 | 0 3 | 23 | 21 | 0 4 | 9 | 7 | 0 5 | 164 | 106 | 0 (6 rows) Mark Kirkwood markir@paradise.net.nz Design suggestions: Neil Conway neilc@samurai.com Debugging advice: Tom Lane tgl@sss.pgh.pa.us