1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
|
<!-- doc/src/sgml/pgfreespacemap.sgml -->
<sect1 id="pgfreespacemap" xreflabel="pg_freespacemap">
<title>pg_freespacemap</title>
<indexterm zone="pgfreespacemap">
<primary>pg_freespacemap</primary>
</indexterm>
<para>
The <filename>pg_freespacemap</filename> module provides a means for examining the
free space map (FSM). It provides a function called
<function>pg_freespace</function>, 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.
</para>
<para>
By default use is restricted to superusers and members of the
<literal>pg_stat_scan_tables</literal> role. Access may be granted to others
using <command>GRANT</command>.
</para>
<sect2>
<title>Functions</title>
<variablelist>
<varlistentry>
<term>
<function>pg_freespace(rel regclass IN, blkno bigint IN) returns int2</function>
<indexterm>
<primary>pg_freespace</primary>
</indexterm>
</term>
<listitem>
<para>
Returns the amount of free space on the page of the relation, specified
by <literal>blkno</literal>, according to the FSM.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
<function>pg_freespace(rel regclass IN, blkno OUT bigint, avail OUT int2)</function>
</term>
<listitem>
<para>
Displays the amount of free space on each page of the relation,
according to the FSM. A set of <literal>(blkno bigint, avail int2)</literal>
tuples is returned, one tuple for each page in the relation.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The values stored in the free space map are not exact. They're rounded
to precision of 1/256th of <symbol>BLCKSZ</symbol> (32 bytes with default <symbol>BLCKSZ</symbol>), and
they're not kept fully up-to-date as tuples are inserted and updated.
</para>
<para>
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.
</para>
</sect2>
<sect2>
<title>Sample Output</title>
<screen>
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)
</screen>
</sect2>
<sect2>
<title>Author</title>
<para>
Original version by Mark Kirkwood <email>markir@paradise.net.nz</email>.
Rewritten in version 8.4 to suit new FSM implementation by Heikki
Linnakangas <email>heikki@enterprisedb.com</email>
</para>
</sect2>
</sect1>
|
