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