Pg_freespacemap - Real time queries on the free space map (FSM).
---------------
This module consists of two C functions: 'pg_freespacemap_relations()' and
'pg_freespacemap_pages()' that return a set of records, plus two views
'pg_freespacemap_relations' and 'pg_freespacemap_pages' for more
user-friendly access to the functions.
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 the functions and views, just in
case there are security issues present in the code.
Installation
------------
Build and install the main Postgresql source, then this contrib module:
$ cd contrib/pg_freespacemap
$ gmake
$ gmake install
To register the functions and views:
$ psql -d <database> -f pg_freespacemap.sql
Notes
-----
The definitions for the columns exposed in the views are:
pg_freespacemap_relations
Column | references | Description
------------------+----------------------+----------------------------------
reltablespace | pg_tablespace.oid | Tablespace oid of the relation.
reldatabase | pg_database.oid | Database oid of the relation.
relfilenode | pg_class.relfilenode | Relfilenode of the relation.
avgrequest | | Moving average of free space
| | requests (NULL for indexes)
interestingpages | | Count of pages last reported as
| | containing useful free space.
storedpages | | Count of pages actually stored
| | in free space map.
nextpage | | Page index (from 0) to start next
| | search at.
pg_freespacemap_pages
Column | references | Description
----------------+----------------------+------------------------------------
reltablespace | pg_tablespace.oid | Tablespace oid of the relation.
reldatabase | pg_database.oid | Database oid of the relation.
relfilenode | pg_class.relfilenode | Relfilenode of the relation.
relblocknumber | | Page number in the relation.
bytes | | Free bytes in the page, or NULL
| | for an index page (see below).
For pg_freespacemap_relations, there is one row for each relation in the free
space map. storedpages is the number of pages actually stored in the map,
while interestingpages is the number of pages the last VACUUM thought had
useful amounts of free space.
If storedpages is consistently less than interestingpages then it'd be a
good idea to increase max_fsm_pages. Also, if the number of rows in
pg_freespacemap_relations is close to max_fsm_relations, then you should
consider increasing max_fsm_relations.
For pg_freespacemap_pages, there is one row for each page in the free space
map. The number of rows for a relation will match the storedpages column
in pg_freespacemap_relations.
For indexes, what is tracked is entirely-unused pages, rather than free
space within pages. Therefore, the average request size and free bytes
within a page are not meaningful, and are shown as NULL.
Because the map is shared by all the databases, it will include relations
not belonging to the current database.
When either of the views are accessed, internal free space map locks are
taken, and a copy of the map data is made for them to display.
This ensures that the views produce a consistent set of results, while not
blocking normal activity longer than necessary. Nonetheless there
could be some impact on database performance if they are read often.
Sample output - pg_freespacemap_relations
-------------
regression=# \d pg_freespacemap_relations
View "public.pg_freespacemap_relations"
Column | Type | Modifiers
------------------+---------+-----------
reltablespace | oid |
reldatabase | oid |
relfilenode | oid |
avgrequest | integer |
interestingpages | integer |
storedpages | integer |
nextpage | integer |
View definition:
SELECT p.reltablespace, p.reldatabase, p.relfilenode, p.avgrequest, p.interestingpages, p.storedpages, p.nextpage
FROM pg_freespacemap_relations() p(reltablespace oid, reldatabase oid, relfilenode oid, avgrequest integer, interestingpages integer, storedpages integer, nextpage integer);
regression=# SELECT c.relname, r.avgrequest, r.interestingpages, r.storedpages
FROM pg_freespacemap_relations r INNER JOIN pg_class c
ON c.relfilenode = r.relfilenode INNER JOIN pg_database d
ON r.reldatabase = d.oid AND (d.datname = current_database())
ORDER BY r.storedpages DESC LIMIT 10;
relname | avgrequest | interestingpages | storedpages
---------------------------------+------------+------------------+-------------
onek | 256 | 109 | 109
pg_attribute | 167 | 93 | 93
pg_class | 191 | 49 | 49
pg_attribute_relid_attnam_index | | 48 | 48
onek2 | 256 | 37 | 37
pg_depend | 95 | 26 | 26
pg_type | 199 | 16 | 16
pg_rewrite | 1011 | 13 | 13
pg_class_relname_nsp_index | | 10 | 10
pg_proc | 302 | 8 | 8
(10 rows)
Sample output - pg_freespacemap_pages
-------------
regression=# \d pg_freespacemap_pages
View "public.pg_freespacemap_pages"
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_pages() p(reltablespace oid, reldatabase oid, relfilenode oid, relblocknumber bigint, bytes integer);
regression=# SELECT c.relname, p.relblocknumber, p.bytes
FROM pg_freespacemap_pages p INNER JOIN pg_class c
ON c.relfilenode = p.relfilenode INNER JOIN pg_database d
ON (p.reldatabase = d.oid AND d.datname = current_database())
ORDER BY c.relname LIMIT 10;
relname | relblocknumber | bytes
--------------+----------------+-------
a_star | 0 | 8040
abstime_tbl | 0 | 7908
aggtest | 0 | 8008
altinhoid | 0 | 8128
altstartwith | 0 | 8128
arrtest | 0 | 7172
b_star | 0 | 7976
box_tbl | 0 | 7912
bt_f8_heap | 54 | 7728
bt_i4_heap | 49 | 8008
(10 rows)
Author
------
* Mark Kirkwood <markir@paradise.net.nz>
9e936693a9