module provides a means for
examining what's happening in the shared buffer cache in real time.
The module provides a C function
that returns a set of records, plus a view
that wraps the function for
By default, use is restricted to superusers and members of the
role. Access may be granted to others
The definitions of the columns exposed by the view are shown in Table F.15 .
ID, in the range 1..
Filenode number of the relation
Tablespace OID of the relation
Database OID of the relation
Fork number within the relation; see
Page number within the relation
Is the page dirty?
Clock-sweep access count
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
. 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
some rows, or that there could even be incorrect joins. If you are
trying to join against
, it's a good idea to
restrict the join to rows having
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
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.24.2. 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)