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.

Name	Type	Description
bufferid	integer	ID, in the range 1..shared_buffers
relfilenode	oid	Filenode number of the relation
reltablespace	oid	Tablespace OID of the relation
reldatabase	oid	Database OID of the relation
relforknumber	smallint	Fork number within the relation; see include/common/relpath.h
relblocknumber	bigint	Page number within the relation
isdirty	boolean	Is the page dirty?
usagecount	smallint	Clock-sweep access count
pinning_backends	integer	Number 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

nijam=#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)