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99 lines
3.5 KiB
Plaintext
99 lines
3.5 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 a C function 'pg_freespacemap()' that returns
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a set of records, and a view 'pg_freespacemap' to wrapper the function.
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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 both of these, just in case there
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are security issues lurking.
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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:
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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 definition of the columns exposed in the view is:
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Column | references | Description
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----------------+----------------------+------------------------------------
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blockid | | Id, 1.. max_fsm_pages
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relfilenode | pg_class.relfilenode | Refilenode of the relation.
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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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relblocknumber | | Offset of the page in the relation.
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blockfreebytes | | Free bytes in the block/page.
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There is one row for each page in the free space map.
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Because the map is shared by all the databases, there are pages from
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relations not belonging to the current database.
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When the pg_freespacemap view is accessed, internal free space map locks are
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taken, and a copy of the map data is made for the view to display.
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This ensures that the view produces 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 this view is read often.
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Sample output
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-------------
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regression=# \d pg_freespacemap
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View "public.pg_freespacemap"
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Column | Type | Modifiers
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---------------+---------+-----------
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blockid | integer |
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relfilenode | oid |
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reltablespace | oid |
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reldatabase | oid |
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relblocknumber | bigint |
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blockfreebytes | integer |
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View definition:
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SELECT p.blockid, p.relfilenode, p.reltablespace, p.reldatabase, p.relblocknumber, p.blockfreebytes
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FROM pg_freespacemap() p(blockid integer, relfilenode oid, reltablespace oid, reldatabase oid, relblocknumber bigint, blockfreebytes integer);
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regression=# SELECT c.relname, m.relblocknumber, m.blockfreebytes
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FROM pg_freespacemap m INNER JOIN pg_class c
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ON c.relfilenode = m.relfilenode LIMIT 10;
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relname | relblocknumber | blockfreebytes
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------------------------+----------------+----------------
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sql_features | 5 | 2696
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sql_implementation_info | 0 | 7104
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sql_languages | 0 | 8016
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sql_packages | 0 | 7376
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sql_sizing | 0 | 6032
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pg_authid | 0 | 7424
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pg_toast_2618 | 13 | 4588
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pg_toast_2618 | 12 | 1680
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pg_toast_2618 | 10 | 1436
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pg_toast_2618 | 7 | 1136
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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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