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116 lines
3.9 KiB
Plaintext
116 lines
3.9 KiB
Plaintext
Pg_buffercache - Real time queries on the shared buffer cache.
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--------------
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This module consists of a C function 'pg_buffercache_pages()' that returns
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a set of records, plus a view 'pg_buffercache' to wrapper the function.
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The intent is to do for the buffercache what pg_locks does for locks, i.e -
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ability to examine what is happening at any given time without having to
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restart or rebuild the server with debugging code added.
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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_buffercache
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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_buffercache.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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bufferid | | Id, 1..shared_buffers.
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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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isdirty | | Is the page dirty?
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usagecount | | Page LRU count
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There is one row for each buffer in the shared cache. Unused buffers are
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shown with all fields null except bufferid.
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Because the cache 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_buffercache view is accessed, internal buffer manager locks are
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taken, and a copy of the buffer cache 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 buffer 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_buffercache;
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View "public.pg_buffercache"
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Column | Type | Modifiers
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----------------+----------+-----------
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bufferid | 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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isdirty | boolean |
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usagecount | smallint |
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View definition:
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SELECT p.bufferid, p.relfilenode, p.reltablespace, p.reldatabase,
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p.relblocknumber, p.isdirty, p.usagecount
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FROM pg_buffercache_pages() p(bufferid integer, relfilenode oid,
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reltablespace oid, reldatabase oid, relblocknumber bigint,
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isdirty boolean, usagecount smallint);
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regression=# SELECT c.relname, count(*) AS buffers
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FROM pg_class c INNER JOIN pg_buffercache b
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ON b.relfilenode = c.relfilenode INNER JOIN pg_database d
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ON (b.reldatabase = d.oid AND d.datname = current_database())
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GROUP BY c.relname
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ORDER BY 2 DESC LIMIT 10;
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relname | buffers
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---------------------------------+---------
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tenk2 | 345
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tenk1 | 141
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pg_proc | 46
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pg_class | 45
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pg_attribute | 43
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pg_class_relname_nsp_index | 30
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pg_proc_proname_args_nsp_index | 28
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pg_attribute_relid_attnam_index | 26
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pg_depend | 22
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pg_depend_reference_index | 20
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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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Help
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----
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* Design suggestions : Neil Conway <neilc@samurai.com>
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* Debugging advice : Tom Lane <tgl@sss.pgh.pa.us>
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Thanks guys!
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