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F.25.1. Thepg_buffercache View

F.25.2. Thepg_buffercache_numa View

F.25.3. Thepg_buffercache_summary() Function

F.25.4. Thepg_buffercache_usage_counts() Function

F.25.5. Thepg_buffercache_evict() Function

F.25.6. Thepg_buffercache_evict_relation() Function

F.25.7. Thepg_buffercache_evict_all() Function

F.25.8. Sample Output

F.25.9. Authors

The definitions of the columns exposed by the view are shown inTable F.14.

There is one row for each buffer in the shared cache. Unused buffers are shown with all fields null exceptbufferid. 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 inpg_class for some rows, or that there could even be incorrect joins. If you are trying to join againstpg_class, it's a good idea to restrict the join to rows havingreldatabase 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, accessingpg_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 definitions of the columns exposed by the view are shown inTable F.15.

AsNUMA node ID inquiry for each page requires memory pages to be paged-in, the first execution of this function can take a noticeable amount of time. In all the cases (first execution or not), retrieving this information is costly and querying the view at a high frequency is not recommended.

The definitions of the columns exposed by the function are shown inTable F.16.

Thepg_buffercache_summary() function returns a single row summarizing the state of all shared buffers. Similar and more detailed information is provided by thepg_buffercache view, butpg_buffercache_summary() is significantly cheaper.

Like thepg_buffercache view,pg_buffercache_summary() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result.

The definitions of the columns exposed by the function are shown inTable F.17.

Thepg_buffercache_usage_counts() function returns a set of rows summarizing the states of all shared buffers, aggregated over the possible usage count values. Similar and more detailed information is provided by thepg_buffercache view, butpg_buffercache_usage_counts() is significantly cheaper.

Like thepg_buffercache view,pg_buffercache_usage_counts() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result.

Thepg_buffercache_evict() function takes a buffer identifier, as shown in thebufferid column of thepg_buffercache view. It returns information about whether the buffer was evicted and flushed. The buffer_evicted column is true on success, and false if the buffer wasn't valid, if it couldn't be evicted because it was pinned, or if it became dirty again after an attempt to write it out. The buffer_flushed column is true if the buffer was flushed. This does not necessarily mean that buffer was flushed by us, it might be flushed by someone else. The result is immediately out of date upon return, as the buffer might become valid again at any time due to concurrent activity. The function is intended for developer testing only.

Thepg_buffercache_evict_relation() function is very similar to thepg_buffercache_evict() function. The difference is that thepg_buffercache_evict_relation() takes a relation identifier instead of buffer identifier. It tries to evict all buffers for all forks in that relation. It returns the number of evicted buffers, flushed buffers and the number of buffers that could not be evicted. Flushed buffers haven't necessarily been flushed by us, they might have been flushed by someone else. The result is immediately out of date upon return, as buffers might immediately be read back in due to concurrent activity. The function is intended for developer testing only.

Thepg_buffercache_evict_all() function is very similar to thepg_buffercache_evict() function. The difference is, thepg_buffercache_evict_all() function does not take an argument; instead it tries to evict all buffers in the buffer pool. It returns the number of evicted buffers, flushed buffers and the number of buffers that could not be evicted. Flushed buffers haven't necessarily been flushed by us, they might have been flushed by someone else. The result is immediately out of date upon return, as buffers might immediately be read back in due to concurrent activity. The function is intended for developer testing only.

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)regression=# SELECT * FROM pg_buffercache_summary(); buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg--------------+----------------+---------------+----------------+----------------          248 |        2096904 |            39 |              0 |       3.141129(1 row)regression=# SELECT * FROM pg_buffercache_usage_counts(); usage_count | buffers | dirty | pinned-------------+---------+-------+--------           0 |   14650 |     0 |      0           1 |    1436 |   671 |      0           2 |     102 |    88 |      0           3 |      23 |    21 |      0           4 |       9 |     7 |      0           5 |     164 |   106 |      0(6 rows)

Mark Kirkwood<markir@paradise.net.nz>

Design suggestions: Neil Conway<neilc@samurai.com>

Debugging advice: Tom Lane<tgl@sss.pgh.pa.us>