PostgreSQL

F.29. pg_freespacemap — examine the free space map
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F.29. pg_freespacemap — examine the free space map #

The pg_freespacemap module provides a means for examining the free space map (FSM). It provides a function called pg_freespace, or two overloaded functions, to be precise. The functions show the value recorded in the free space map for a given page, or for all pages in the relation.

By default use is restricted to superusers and roles with privileges of the pg_stat_scan_tables role. Access may be granted to others using GRANT.

F.29.1. Functions #

[.term]#pg_freespace(rel regclass IN, blkno bigint IN) returns int2: Returns the amount of free space on the page of the relation, specified by blkno, according to the FSM.
pg_freespace(rel regclass IN, blkno OUT bigint, avail OUT int2): Displays the amount of free space on each page of the relation, according to the FSM. A set of (blkno bigint, avail int2) tuples is returned, one tuple for each page in the relation.

The values stored in the free space map are not exact. They’re rounded to precision of 1/256th of BLCKSZ (32 bytes with default BLCKSZ), and they’re not kept fully up-to-date as tuples are inserted and updated.

For indexes, what is tracked is entirely-unused pages, rather than free space within pages. Therefore, the values are not meaningful, just whether a page is full or empty.

F.29.2. Sample Output #

postgres=# SELECT * FROM pg_freespace('foo');
 blkno | avail
-------+-------
     0 |     0
     1 |     0
     2 |     0
     3 |    32
     4 |   704
     5 |   704
     6 |   704
     7 |  1216
     8 |   704
     9 |   704
    10 |   704
    11 |   704
    12 |   704
    13 |   704
    14 |   704
    15 |   704
    16 |   704
    17 |   704
    18 |   704
    19 |  3648
(20 rows)

postgres=# SELECT * FROM pg_freespace('foo', 7);
 pg_freespace
--------------
         1216
(1 row)

F.29.3. Author #

Original version by Mark Kirkwood <`[email protected]>. Rewritten in version 8.4 to suit new FSM implementation by Heikki Linnakangas `<`[email protected]>`

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