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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>