F.27. pg_buffercache #

F.27.1. The pg_buffercache View
F.27.2. The pg_buffercache_summary() Function
F.27.3. Sample Output
F.27.4. Authors

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

The module provides the pg_buffercache_pages() function, wrapped in the pg_buffercache view, and the pg_buffercache_summary() function.

The pg_buffercache_pages() function returns a set of records, each row describing the state of one shared buffer entry. The pg_buffercache view wraps the function for convenient use.

The pg_buffercache_summary() function returns a single row summarizing the state of the shared buffer cache.

By default, use is restricted to superusers and roles with privileges of the pg_monitor role. Access may be granted to others using GRANT.

F.27.1. The pg_buffercache View #

The definitions of the columns exposed by the view are shown in Table F.15.

Table F.15. pg_buffercache Columns

Column Type

Description

bufferid integer

ID, in the range 1..shared_buffers

relfilenode oid (references pg_class.relfilenode)

Filenode number of the relation

reltablespace oid (references pg_tablespace.oid)

Tablespace OID of the relation

reldatabase oid (references 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


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.27.2. The pg_buffercache_summary() Function #

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

Table F.16. pg_buffercache_summary() Output Columns

Column Type

Description

buffers_used int4

Number of unused shared buffers

buffers_unused int4

Number of unused shared buffers

buffers_dirty int4

Number of dirty shared buffers

buffers_pinned int4

Number of pinned shared buffers

usagecount_avg float8

Average usagecount of used shared buffers


The pg_buffercache_summary() function returns a single row summarizing the state of all shared buffers. Similar and more detailed information is provided by the pg_buffercache view, but pg_buffercache_summary() is significantly cheaper.

Like the pg_buffercache view, pg_buffercache_summary() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result.

F.27.3. 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)


regression=# SELECT * FROM pg_buffercache_summary();
 buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg
--------------+----------------+---------------+----------------+----------------
          248 |        2096904 |            39 |              0 |       3.141129
(1 row)

F.27.4. Authors #

Mark Kirkwood

Design suggestions: Neil Conway

Debugging advice: Tom Lane