F.23. 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.23.1. 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.
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.23.2. 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.23.3. Authors
Mark Kirkwood <markir@paradise.net.nz>
Design suggestions: Neil Conway <neilc@samurai.com>
Debugging advice: Tom Lane <tgl@sss.pgh.pa.us>