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166 lines
6.1 KiB
Plaintext
166 lines
6.1 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 for the relation.
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relfilenode | pg_class.relfilenode | Refilenode of the relation.
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avgrequest | | Moving average of free space
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| | requests.
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lastpagecount | | Count of pages examined for useful
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| | free space.
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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 for the relation.
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relfilenode | pg_class.relfilenode | Refilenode of the relation.
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relblocknumber | | Page offset 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.
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For pg_freespacemap_pages, there is one row for each page in the free space
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map.
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Because the map is shared by all the databases, there are relations and pages
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from relations not belonging to the current database.
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The view 'freespacemap_pages' can contain pages for btree indexes if they
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were emptied by a vacuum process. The bytes field is set to NULL in this case.
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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 | bigint |
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lastpagecount | 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.lastpagecount, p.nextpage
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FROM pg_freespacemap_relations() p(reltablespace oid, reldatabase oid, relfilenode oid, avgrequest bigint, lastpagecount integer, nextpage integer);
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regression=# SELECT c.relname, r.avgrequest, r.lastpagecount, r.nextpage
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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 c.relname LIMIT 10;
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relname | avgrequest | lastpagecount | nextpage
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--------------+------------+---------------+----------
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a_star | 250 | 1 | 0
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abstime_tbl | 249 | 1 | 0
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aggtest | 250 | 1 | 0
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altinhoid | 250 | 1 | 0
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altstartwith | 250 | 1 | 0
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arrtest | 254 | 1 | 0
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b_star | 250 | 1 | 0
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box_tbl | 250 | 1 | 0
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bt_f8_heap | 92 | 1 | 0
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bt_i4_heap | 94 | 1 | 0
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(10 rows)
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regression=#
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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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regression=#
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Author
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------
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* Mark Kirkwood <markir@paradise.net.nz>
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