Request a Demo
F.39. pg_freespacemap
Prev UpAppendix F. Additional Supplied Modules and Extensions Shipped in postgrespro-ent-14-contribHome Next

F.39. pg_freespacemap

F.39.1. Functions
F.39.2. Sample Output
F.39.3. Author

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 members of the pg_stat_scan_tables role. Access may be granted to others using GRANT.

F.39.1. Functions

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.

pg_freespace_relation_logical_blocks(relation regclass [, fork text ]) returns bigint

Computes the disk space used by one fork of the specified relation in blocks (pages). Use the fork parameter to specify which fork to examine. Possible values:

  • main returns the size of the main data fork of the relation.

  • fsm returns the size of the Free Space Map (see Section 70.3) associated with the relation.

  • vm returns the size of the Visibility Map (see Section 70.4) associated with the relation.

Note

The init value will always return 0, as the init fork is not associated with the disk.

If fork is omitted, the function will return the size of the main data fork of the relation.

This function returns correct results independent of whether the table is compressed or not.

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.

F.39.2. 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.39.3. Author

Original version by Mark Kirkwood . Rewritten in version 8.4 to suit new FSM implementation by Heikki Linnakangas

Prev Up Next
F.38. pgcrypto Home F.40. pg_hint_plan
epub pdf