| PostgreSQL 8.3.23 Documentation | ||||
|---|---|---|---|---|
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The pg_freespacemap module provides
a means for examining the free space map (FSM). It provides two C
functions: pg_freespacemap_relations and pg_freespacemap_pages that each return a set of
records, plus two views pg_freespacemap_relations and pg_freespacemap_pages that wrap the functions
for convenient use.
By default public access is revoked from the functions and views, just in case there are security issues lurking.
The definitions of the columns exposed by the views are:
Table F-23. pg_freespacemap_relations Columns
| Name | Type | References | Description |
|---|---|---|---|
| reltablespace | oid | pg_tablespace.oid | Tablespace OID of the relation |
| reldatabase | oid | pg_database.oid | Database OID of the relation |
| relfilenode | oid | pg_class.relfilenode | Relfilenode of the relation |
| avgrequest | integer | Moving average of free space requests (NULL for indexes) | |
| interestingpages | integer | Count of pages last reported as containing useful free space | |
| storedpages | integer | Count of pages actually stored in free space map | |
| nextpage | integer | Page index (from 0) to start next search at |
Table F-24. pg_freespacemap_pages Columns
| Name | Type | References | Description |
|---|---|---|---|
| reltablespace | oid | pg_tablespace.oid | Tablespace OID of the relation |
| reldatabase | oid | pg_database.oid | Database OID of the relation |
| relfilenode | oid | pg_class.relfilenode | Relfilenode of the relation |
| relblocknumber | bigint | Page number within the relation | |
| bytes | integer | Free bytes in the page, or NULL for an index page (see below) |
For pg_freespacemap_relations, there is one row for each relation in the free space map. storedpages is the number of pages actually stored in the map, while interestingpages is the number of pages the last VACUUM thought had useful amounts of free space.
If storedpages is consistently less than interestingpages then it'd be a good idea to increase max_fsm_pages. Also, if the number of rows in pg_freespacemap_relations is close to max_fsm_relations, then you should consider increasing max_fsm_relations.
For pg_freespacemap_pages, there is one row for each page in the free space map. The number of rows for a relation will match the storedpages column in pg_freespacemap_relations.
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.
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 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.
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.
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)
Mark Kirkwood <markir@paradise.net.nz>