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_read_all_stats role. Access may be
granted to others using GRANT.
pg_buffercache ViewThe 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.
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)Mark Kirkwood <markir@paradise.net.nz>
Design suggestions: Neil Conway <neilc@samurai.com>
Debugging advice: Tom Lane <tgl@sss.pgh.pa.us>
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