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Postgresql pg_buffercache

  • The pg_buffercache module provides a means for examining what's happening in the shared buffer cache in real time.
  • The module provides a C function pg_buffercache_pages that returns a set of records, plus a view pg_buffercache that wraps the function for convenient use.
  • By default public access is revoked from both of these, just in case there are security issues lurking.
bufferidintegerID, in the range 1..shared_buffers
relfilenodeoidpg_class.relfilenodeFilenode number of the relation
reltablespaceoidpg_tablespace.oidTablespace OID of the relation
reldatabaseoidpg_database.oidDatabase OID of the relation
relforknumbersmallintFork number within the relation; see include/common/relpath.h
relblocknumberbigintPage number within the relation
isdirtybooleanIs the page dirty?
usagecountsmallintClock-sweep access count
pinning_backendsintegerNumber of backends pinning this buffer
  • There is one row for each buffer in the shared cache. Unused buffers are shown with all fields null except bufferid. Shared system catalogs are shown as belonging to database zero.
  • Because the cache is shared by all the databases, there will normally be pages from relations not belonging to the current database. This means that there may not be matching join rows in pg_class for some rows, or that there could even be incorrect joins. If you are trying to join against pg_class, it's a good idea to restrict the join to rows having reldatabase equal to the current database's OID or zero.
  • When the pg_buffercache view is accessed, internal buffer manager locks are taken for long enough to copy all the buffer state data that the view will display. This ensures that the view produces a consistent set of results, while not blocking normal buffer activity longer than necessary. Nonetheless there could be some impact on database performance if this view is read often
  • Example For
regression=#SELECT c.relname, count(*) AS buffers FROM pg_buffercache b INNER JOIN pg_class c ON b.relfilenode = pg_relation_filenode(c.oid) AND b.reldatabase IN (0, (SELECT oid FROM pg_database WHERE datname = current_database())) GROUP BY c.relname ORDER BY 2 DESC LIMIT 10;
             relname             | buffers
 tenk2                           |     345
 tenk1                           |     141
 pg_proc                         |      46
 pg_class                        |      45
 pg_attribute                    |      43
 pg_class_relname_nsp_index      |      30
 pg_proc_proname_args_nsp_index  |      28
 pg_attribute_relid_attnam_index |      26
 pg_depend                       |      22
 pg_depend_reference_index       |      20
(10 rows)


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