pg\_buffercache {#pgbuffercache}
===============

pg\_buffercache

The `pg_buffercache` module provides a means for examining what\'s
happening in the shared buffer cache in real time.

pg\_buffercache\_pages

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 roles with privileges of
the `pg_monitor` role. Access may be granted to others using `GRANT`.

The pg\_buffercache View
------------------------

The definitions of the columns exposed by the view are shown in
[table\_title](#pgbuffercache-columns).

+-----------------------------------------------------------------------+
| Column Type                                                           |
|                                                                       |
| Description                                                           |
+=======================================================================+
| bufferid `integer`                                                    |
|                                                                       |
| ID, in the range 1..`shared_buffers`                                  |
+-----------------------------------------------------------------------+
| relfilenode `oid` (references                                         |
| [pg\_class](#catalog-pg-class).relfilenode)                           |
|                                                                       |
| Filenode number of the relation                                       |
+-----------------------------------------------------------------------+
| reltablespace `oid` (references                                       |
| [pg\_tablespace](#catalog-pg-tablespace).oid)                         |
|                                                                       |
| Tablespace OID of the relation                                        |
+-----------------------------------------------------------------------+
| reldatabase `oid` (references                                         |
| [pg\_database](#catalog-pg-database).oid)                             |
|                                                                       |
| Database OID of the relation                                          |
+-----------------------------------------------------------------------+
| relforknumber `smallint`                                              |
|                                                                       |
| Fork number within the relation; see `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                                |
+-----------------------------------------------------------------------+

: pg\_buffercache Columns

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.

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)

Authors
-------

Mark Kirkwood <markir@paradise.net.nz>

Design suggestions: Neil Conway <neilc@samurai.com>

Debugging advice: Tom Lane <tgl@sss.pgh.pa.us>
