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><A
NAME="PGFREESPACEMAP"
>F.26. pg_freespacemap</A
></H1
><P
> The <TT
CLASS="FILENAME"
>pg_freespacemap</TT
> module provides a means for examining the
free space map (FSM). It provides a function called
<CODE
CLASS="FUNCTION"
>pg_freespace</CODE
>, 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.
</P
><P
> By default public access is revoked from the functions, just in case
there are security issues lurking.
</P
><DIV
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><H2
CLASS="SECT2"
><A
NAME="AEN150875"
>F.26.1. Functions</A
></H2
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><DL
><DT
><CODE
CLASS="FUNCTION"
>pg_freespace(rel regclass IN, blkno bigint IN) returns int2</CODE
></DT
><DD
><P
> Returns the amount of free space on the page of the relation, specified
by <TT
CLASS="LITERAL"
>blkno</TT
>, according to the FSM.
</P
></DD
><DT
><CODE
CLASS="FUNCTION"
>pg_freespace(rel regclass IN, blkno OUT bigint, avail OUT int2)</CODE
></DT
><DD
><P
> Displays the amount of free space on each page of the relation,
according to the FSM. A set of <TT
CLASS="LITERAL"
>(blkno bigint, avail int2)</TT
>
tuples is returned, one tuple for each page in the relation.
</P
></DD
></DL
></DIV
><P
> The values stored in the free space map are not exact. They're rounded
to precision of 1/256th of <TT
CLASS="SYMBOL"
>BLCKSZ</TT
> (32 bytes with default <TT
CLASS="SYMBOL"
>BLCKSZ</TT
>), and
they're not kept fully up-to-date as tuples are inserted and updated.
</P
><P
> 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.
</P
><P
> NOTE: The interface was changed in version 8.4, to reflect the new FSM
implementation introduced in the same version.
</P
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><H2
CLASS="SECT2"
><A
NAME="AEN150895"
>F.26.2. Sample Output</A
></H2
><PRE
CLASS="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)</PRE
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><H2
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><A
NAME="AEN150898"
>F.26.3. Author</A
></H2
><P
> Original version by Mark Kirkwood <CODE
CLASS="EMAIL"
><<A
HREF="mailto:markir@paradise.net.nz"
>markir@paradise.net.nz</A
>></CODE
>.
Rewritten in version 8.4 to suit new FSM implementation by Heikki
Linnakangas <CODE
CLASS="EMAIL"
><<A
HREF="mailto:heikki@enterprisedb.com"
>heikki@enterprisedb.com</A
>></CODE
>
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