F.27. 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.27.1. 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.
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.27.2. 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.27.3. Author #
Original version by Mark Kirkwood
<
markir@paradise.net.nz
>
.
Rewritten in version 8.4 to suit new
FSM
implementation
by Heikki Linnakangas
<
heikki@enterprisedb.com
>