F.35. pg_buffercache — inspect Postgres Pro buffer cache state #
The pg_buffercache module provides a means for examining what's happening in the shared buffer cache in real time. It also offers a low-level way to evict data from it, for testing purposes.
This module provides the pg_buffercache_pages() function (wrapped in the pg_buffercache view), the pg_buffercache_summary() function, the pg_buffercache_usage_counts() function and the pg_buffercache_evict() 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 of the above functions is restricted to superusers and roles with privileges of the pg_monitor role. Access may be granted to others using GRANT.
The pg_buffercache_evict() function allows a block to be evicted from the buffer pool given a buffer identifier. Use of this function is restricted to superusers only.
F.35.1. The pg_buffercache View #
The definitions of the columns exposed by the view are shown in Table F.19.
Table F.19. pg_buffercache Columns
Column Type Description |
|---|
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 |
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 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.35.2. The pg_buffercache_summary() Function #
The definitions of the columns exposed by the function are shown in Table F.20.
Table F.20. pg_buffercache_summary() Output Columns
Column Type Description |
|---|
Number of used shared buffers |
Number of unused shared buffers |
Number of dirty shared buffers |
Number of pinned shared buffers |
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.
F.35.3. The pg_buffercache_usage_counts() Function #
The definitions of the columns exposed by the function are shown in Table F.21.
Table F.21. pg_buffercache_usage_counts() Output Columns
Column Type Description |
|---|
A possible buffer usage count |
Number of buffers with the usage count |
Number of dirty buffers with the usage count |
Number of pinned buffers with the usage count |
The pg_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 the pg_buffercache view, but pg_buffercache_usage_counts() is significantly cheaper.
Like the pg_buffercache view, pg_buffercache_usage_counts() does not acquire buffer manager locks. Therefore concurrent activity can lead to minor inaccuracies in the result.
F.35.4. The pg_buffercache_evict Function #
The pg_buffercache_evict() function takes a buffer identifier, as shown in the bufferid column of the pg_buffercache view. It returns 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 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.
F.35.5. 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)
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)
F.35.6. Authors #
Mark Kirkwood <markir@paradise.net.nz>
Design suggestions: Neil Conway <neilc@samurai.com>
Debugging advice: Tom Lane <tgl@sss.pgh.pa.us>