pgfreespacemap
pgfreespacemap
This modules provides the means for examining the free space map (FSM). It
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.
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