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F.27. 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.

F.27.1. 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
relblocknumber bigint   Page number within the relation
relforknumber smallint   Fork number within the relation
isdirty boolean   Is the page dirty?
usagecount smallint   Page LRU count

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.

F.27.2. Sample Output

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)

F.27.3. Authors

Mark Kirkwood

Design suggestions: Neil Conway

Debugging advice: Tom Lane