F.25. pg_buffercache
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## F.25. 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, use is restricted to superusers and members of the `pg_monitor` role. Access may be granted to others using `GRANT`.

### F.25.1. 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.

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.25.2. 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.25.3. Authors

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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