pg_buffercache — inspect <productname>PostgreSQL</productname> buffer cache state

pg_buffercache — inspect <productname>PostgreSQL</productname> buffer cache state pg_buffercache The pg_buffercache module provides a means for examining what's happening in the shared buffer cache in real time. pg_buffercache_pages pg_buffercache_summary This module provides the pg_buffercache_pages() function (wrapped in the pg_buffercache view), the pg_buffercache_summary() function, and the pg_buffercache_usage_counts() function. The pg_buffercache_pages() function returns a set of records, each row describing the state of one shared buffer entry. The pg_buffercache view wraps the function for convenient use. The pg_buffercache_summary() function returns a single row summarizing the state of the shared buffer cache. The pg_buffercache_usage_counts() function returns a set of records, each row describing the number of buffers with a given usage count. By default, use is restricted to superusers and roles with privileges of the pg_monitor role. Access may be granted to others using GRANT. The <structname>pg_buffercache</structname> View The definitions of the columns exposed by the view are shown in . <structname>pg_buffercache</structname> 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 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. The <function>pg_buffercache_summary()</function> Function The definitions of the columns exposed by the function are shown in . <function>pg_buffercache_summary()</function> Output Columns Column Type Description buffers_used int4 Number of used shared buffers buffers_unused int4 Number of unused shared buffers buffers_dirty int4 Number of dirty shared buffers buffers_pinned int4 Number of pinned shared buffers usagecount_avg float8 Average usage count of used shared buffers

The pg_buffercache_summary() function returns a single row summarizing the state of all shared buffers. Similar and more detailed information is provided by the pg_buffercache view, but pg_buffercache_summary() is significantly cheaper. Like the pg_buffercache view, pg_buffercache_summary() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result. The <function>pg_buffercache_usage_counts()</function> Function The definitions of the columns exposed by the function are shown in . <function>pg_buffercache_usage_counts()</function> Output Columns Column Type Description usage_count int4 A possible buffer usage count buffers int4 Number of buffers with the usage count dirty int4 Number of dirty buffers with the usage count pinned int4 Number of pinned buffers with the usage count