pg_freespacemap

PostgreSQL 8.3.3 Documentation
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# F.21. pg_freespacemap

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.

## F.21.1. The `pg_freespacemap` views

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.

## F.21.2. Sample output

    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)

## F.21.3. Author

Mark Kirkwood `<markir@paradise.net.nz>`

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