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 for the relation.
relfilenode | pg_class.relfilenode | Refilenode of the relation.
avgrequest | | Moving average of free space
| | requests.
lastpagecount | | Count of pages examined for useful
| | free space.
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 for the relation.
relfilenode | pg_class.relfilenode | Refilenode of the relation.
relblocknumber | | Page offset 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.
For pg_freespacemap_pages, there is one row for each page in the free space
map.
Because the map is shared by all the databases, there are relations and pages
from relations not belonging to the current database.
The view 'freespacemap_pages' 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 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 | bigint |
lastpagecount | integer |
nextpage | integer |
View definition:
SELECT p.reltablespace, p.reldatabase, p.relfilenode, p.avgrequest, p.lastpagecount, p.nextpage
FROM pg_freespacemap_relations() p(reltablespace oid, reldatabase oid, relfilenode oid, avgrequest bigint, lastpagecount integer, nextpage integer);
regression=# SELECT c.relname, r.avgrequest, r.lastpagecount, r.nextpage
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 c.relname LIMIT 10;
relname | avgrequest | lastpagecount | nextpage
--------------+------------+---------------+----------
a_star | 250 | 1 | 0
abstime_tbl | 249 | 1 | 0
aggtest | 250 | 1 | 0
altinhoid | 250 | 1 | 0
altstartwith | 250 | 1 | 0
arrtest | 254 | 1 | 0
b_star | 250 | 1 | 0
box_tbl | 250 | 1 | 0
bt_f8_heap | 92 | 1 | 0
bt_i4_heap | 94 | 1 | 0
(10 rows)
regression=#
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)
regression=#
Author
------
* Mark Kirkwood <markir@paradise.net.nz>
136bea1540