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F.25. 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, use is restricted to superusers and members of the pg_monitor role. Access may be granted to others using GRANT.

F.25.1. The pg_buffercache View

The definitions of the columns exposed by the view are shown in Table F.16.

Table F.16. pg_buffercache Columns

Name Type References Description
bufferid integer   ID, in the range 1..shared_buffers
relfilenode oid pg_class.relfilenode Filenode number of the relation
reltablespace oid pg_tablespace.oid Tablespace OID of the relation
reldatabase oid pg_database.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.

Since buffer manager locks are not taken to copy the buffer state data that the view will display, accessing pg_buffercache view has less impact on normal buffer activity but it doesn't provide a consistent set of results across all buffers. However, we ensure that the information of each buffer is self-consistent.

F.25.2. Sample Output

regression=# SELECT n.nspname, c.relname, count(*) AS buffers
             FROM pg_buffercache b 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()))
             JOIN pg_namespace n ON n.oid = c.relnamespace
             GROUP BY n.nspname, c.relname
             ORDER BY 3 DESC
             LIMIT 10;

  nspname   |        relname         | buffers
 public     | delete_test_table      |     593
 public     | delete_test_table_pkey |     494
 pg_catalog | pg_attribute           |     472
 public     | quad_poly_tbl          |     353
 public     | tenk2                  |     349
 public     | tenk1                  |     349
 public     | gin_test_idx           |     306
 pg_catalog | pg_largeobject         |     206
 public     | gin_test_tbl           |     188
 public     | spgist_text_tbl        |     182
(10 rows)

F.25.3. Authors

Mark Kirkwood

Design suggestions: Neil Conway

Debugging advice: Tom Lane