Pg_freespacemap - Real time queries on the free space map (FSM).
---------------
This module consists of a C function 'pg_freespacemap()' that returns
a set of records, and a view 'pg_freespacemap' to wrapper the function.
The module provides the ability to examine the contents of the free space
map, without having to restart or rebuild the server with additional
debugging code.
By default public access is REVOKED from both of these, just in case there
are security issues lurking.
Installation
------------
Build and install the main Postgresql source, then this contrib module:
$ cd contrib/pg_freespacemap
$ gmake
$ gmake install
To register the functions:
$ psql -d <database> -f pg_freespacemap.sql
Notes
-----
The definition of the columns exposed in the view is:
Column | references | Description
----------------+----------------------+------------------------------------
reltablespace | pg_tablespace.oid | Tablespace oid of the relation.
reldatabase | pg_database.oid | Database for the relation.
relfilenode | pg_class.relfilenode | Refilenode of the relation.
relblocknumber | | Offset of the page in the relation.
bytes | | Free bytes in the block/page, or NULL
| | for an index page (see below).
There is one row for each page in the free space map.
Because the map is shared by all the databases, there are pages from
relations not belonging to the current database.
The free space map can contain pages for btree indexes if they were emptied
by a vacuum process. The bytes field is set to NULL in this case.
When the pg_freespacemap view is accessed, internal free space map locks are
taken, and a copy of the map data is made for the view to display.
This ensures that the view produces a consistent set of results, while not
blocking normal activity longer than necessary. Nonetheless there
could be some impact on database performance if this view is read often.
Sample output
-------------
regression=# \d pg_freespacemap
View "public.pg_freespacemap"
Column | Type | Modifiers
----------------+---------+-----------
reltablespace | oid |
reldatabase | oid |
relfilenode | oid |
relblocknumber | bigint |
bytes | integer |
View definition:
SELECT p.reltablespace, p.reldatabase, p.relfilenode, p.relblocknumber, p.bytes
FROM pg_freespacemap() p(reltablespace oid, reldatabase oid, relfilenode oid, relblocknumber bigint, bytes integer);
regression=# SELECT c.relname, m.relblocknumber, m.bytes
FROM pg_freespacemap m INNER JOIN pg_class c
ON c.relfilenode = m.relfilenode LIMIT 10;
relname | relblocknumber | bytes
------------------------+----------------+--------
sql_features | 5 | 2696
sql_implementation_info | 0 | 7104
sql_languages | 0 | 8016
sql_packages | 0 | 7376
sql_sizing | 0 | 6032
pg_authid | 0 | 7424
pg_toast_2618 | 13 | 4588
pg_toast_2618 | 12 | 1680
pg_toast_2618 | 10 | 1436
pg_toast_2618 | 7 | 1136
(10 rows)
regression=#
Author
------
* Mark Kirkwood <markir@paradise.net.nz>
028ec5cb0f