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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.
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 in-use or empty.
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)
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