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<!-- $PostgreSQL: pgsql/doc/src/sgml/pgfreespacemap.sgml,v 2.2 2007/12/10 05:32:51 tgl Exp $ -->
<sect1 id="pgfreespacemap">
<title>pg_freespacemap</title>
<indexterm zone="pgfreespacemap">
<primary>pg_freespacemap</primary>
</indexterm>
<para>
The <filename>pg_freespacemap</> module provides a means for examining the
free space map (FSM). It provides two C functions:
<function>pg_freespacemap_relations</function> and
<function>pg_freespacemap_pages</function> that each return a set of
records, plus two views <structname>pg_freespacemap_relations</structname>
and <structname>pg_freespacemap_pages</structname> that wrap the functions
for convenient use.
</para>
<para>
By default public access is revoked from the functions and views, just in
case there are security issues lurking.
</para>
<sect2>
<title>The <filename>pg_freespacemap</> views</title>
<para>
The definitions of the columns exposed by the views are:
</para>
<table>
<title><structname>pg_freespacemap_relations</> Columns</title>
<tgroup cols="4">
<thead>
<row>
<entry>Name</entry>
<entry>Type</entry>
<entry>References</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>reltablespace</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal>pg_tablespace.oid</literal></entry>
<entry>Tablespace OID of the relation</entry>
</row>
<row>
<entry><structfield>reldatabase</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal>pg_database.oid</literal></entry>
<entry>Database OID of the relation</entry>
</row>
<row>
<entry><structfield>relfilenode</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal>pg_class.relfilenode</literal></entry>
<entry>Relfilenode of the relation</entry>
</row>
<row>
<entry><structfield>avgrequest</structfield></entry>
<entry><type>integer</type></entry>
<entry></entry>
<entry>Moving average of free space requests (NULL for indexes)</entry>
</row>
<row>
<entry><structfield>interestingpages</structfield></entry>
<entry><type>integer</type></entry>
<entry></entry>
<entry>Count of pages last reported as containing useful free space</entry>
</row>
<row>
<entry><structfield>storedpages</structfield></entry>
<entry><type>integer</type></entry>
<entry></entry>
<entry>Count of pages actually stored in free space map</entry>
</row>
<row>
<entry><structfield>nextpage</structfield></entry>
<entry><type>integer</type></entry>
<entry></entry>
<entry>Page index (from 0) to start next search at</entry>
</row>
</tbody>
</tgroup>
</table>
<table>
<title><structname>pg_freespacemap_pages</> Columns</title>
<tgroup cols="4">
<thead>
<row>
<entry>Name</entry>
<entry>Type</entry>
<entry>References</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><structfield>reltablespace</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal>pg_tablespace.oid</literal></entry>
<entry>Tablespace OID of the relation</entry>
</row>
<row>
<entry><structfield>reldatabase</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal>pg_database.oid</literal></entry>
<entry>Database OID of the relation</entry>
</row>
<row>
<entry><structfield>relfilenode</structfield></entry>
<entry><type>oid</type></entry>
<entry><literal>pg_class.relfilenode</literal></entry>
<entry>Relfilenode of the relation</entry>
</row>
<row>
<entry><structfield>relblocknumber</structfield></entry>
<entry><type>bigint</type></entry>
<entry></entry>
<entry>Page number within the relation</entry>
</row>
<row>
<entry><structfield>bytes</structfield></entry>
<entry><type>integer</type></entry>
<entry></entry>
<entry>Free bytes in the page, or NULL for an index page (see below)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
For <structname>pg_freespacemap_relations</structname>, there is one row
for each relation in the free space map.
<structfield>storedpages</structfield> is the number of pages actually
stored in the map, while <structfield>interestingpages</structfield> is the
number of pages the last <command>VACUUM</> thought had useful amounts of
free space.
</para>
<para>
If <structfield>storedpages</structfield> is consistently less than
<structfield>interestingpages</> then it'd be a good idea to increase
<varname>max_fsm_pages</varname>. Also, if the number of rows in
<structname>pg_freespacemap_relations</structname> is close to
<varname>max_fsm_relations</varname>, then you should consider increasing
<varname>max_fsm_relations</varname>.
</para>
<para>
For <structname>pg_freespacemap_pages</structname>, there is one row for
each page in the free space map. The number of rows for a relation will
match the <structfield>storedpages</structfield> column in
<structname>pg_freespacemap_relations</structname>.
</para>
<para>
For indexes, what is tracked is entirely-unused pages, rather than free
space within pages. Therefore, the average request size and free bytes
within a page are not meaningful, and are shown as NULL.
</para>
<para>
Because the map is shared by all the databases, there will normally be
entries for relations not belonging to the current database. This means
that there may not be matching join rows in <structname>pg_class</> for
some rows, or that there could even be incorrect joins. If you are
trying to join against <structname>pg_class</>, it's a good idea to
restrict the join to rows having <structfield>reldatabase</> equal to
the current database's OID or zero.
</para>
<para>
When either of the views is accessed, internal free space map locks are
taken for long enough to copy all the state data that the view will display.
This ensures that the views produce a consistent set of results, while not
blocking normal activity longer than necessary. Nonetheless there
could be some impact on database performance if they are read often.
</para>
</sect2>
<sect2>
<title>Sample output</title>
<programlisting>
regression=# SELECT c.relname, r.avgrequest, r.interestingpages, r.storedpages
FROM pg_freespacemap_relations r INNER JOIN pg_class c
ON r.relfilenode = c.relfilenode AND
r.reldatabase IN (0, (SELECT oid FROM pg_database
WHERE datname = current_database()))
ORDER BY r.storedpages DESC LIMIT 10;
relname | avgrequest | interestingpages | storedpages
---------------------------------+------------+------------------+-------------
onek | 256 | 109 | 109
pg_attribute | 167 | 93 | 93
pg_class | 191 | 49 | 49
pg_attribute_relid_attnam_index | | 48 | 48
onek2 | 256 | 37 | 37
pg_depend | 95 | 26 | 26
pg_type | 199 | 16 | 16
pg_rewrite | 1011 | 13 | 13
pg_class_relname_nsp_index | | 10 | 10
pg_proc | 302 | 8 | 8
(10 rows)
regression=# SELECT c.relname, p.relblocknumber, p.bytes
FROM pg_freespacemap_pages p INNER JOIN pg_class c
ON p.relfilenode = c.relfilenode AND
p.reldatabase IN (0, (SELECT oid FROM pg_database
WHERE datname = current_database()))
ORDER BY c.relname LIMIT 10;
relname | relblocknumber | bytes
--------------+----------------+-------
a_star | 0 | 8040
abstime_tbl | 0 | 7908
aggtest | 0 | 8008
altinhoid | 0 | 8128
altstartwith | 0 | 8128
arrtest | 0 | 7172
b_star | 0 | 7976
box_tbl | 0 | 7912
bt_f8_heap | 54 | 7728
bt_i4_heap | 49 | 8008
(10 rows)
</programlisting>
</sect2>
<sect2>
<title>Author</title>
<para>
Mark Kirkwood <email>markir@paradise.net.nz</email>
</para>
</sect2>
</sect1>
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