F.24. pg_buffercache

pg_buffercache 延伸功能提供了一個即時檢查共享緩衝區配置情況的方法。

該模塊提供了一個 C 函數的 pg_buffercache_pages，該函數回傳一組記錄，以及一個檢視表 pg_buffercache，該檢視表封裝了該函數以便於使用。

預設情況下，僅限於超級使用者和 pg_monitor 角色的成員使用。也可以使用 GRANT 將存取權限授予其他角色。

F.24.1. The pg_buffercache View

Table F.15 中列出了此檢視表的欄位定義。

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 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.

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.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)

F.24.3. Authors

Mark Kirkwood <[email protected]>

Design suggestions: Neil Conway <[email protected]>

Debugging advice: Tom Lane <[email protected]>