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even when a single relation requires more than max_fsm_pages pages. Also, make VACUUM emit a warning in this case, since it likely means that VACUUM FULL or other drastic corrective measure is needed. Per reports from Jeff Frost and others of unexpected changes in the claimed max_fsm_pages need.
174 lines
7.2 KiB
Plaintext
174 lines
7.2 KiB
Plaintext
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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