postgresql pg_buffercache
pg_buffercache模块是用于查看shared buffer cache信息，决定shared buffer cache大还是小。
Installing pg_buffercache into a database：
$ createdb pgbench
$ psql -d pgbench -f /usr/share/postgresql/contrib/pg_buffercache.sql
两步即可完成
pg_buffercache.sql内容：
/* contrib/pg_buffercache/pg_buffercache--1.0.sql */
-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION pg_buffercache" to load this file. \quit
-- Register the function.
CREATE FUNCTION pg_buffercache_pages()
RETURNS SETOF RECORD
AS 'MODULE_PATHNAME', 'pg_buffercache_pages'
LANGUAGE C;
-- Create a view for convenient access.
CREATE VIEW pg_buffercache AS
SELECT P.* FROM pg_buffercache_pages() AS P
(bufferid integer, relfilenode oid, reltablespace oid, reldatabase oid,
relforknumber int2, relblocknumber int8, isdirty bool, usagecount int2);
-- Don't want these to be available to public.
REVOKE ALL ON FUNCTION pg_buffercache_pages() FROM PUBLIC;
REVOKE ALL ON pg_buffercache FROM PUBLIC;
创建函数和视图，回收PUBLIC 权限。
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
relblocknumber
bigint
Page number within the relation
relforknumber
smallint
Fork number within the relation
isdirty
boolean
Is the page dirty
usagecount
smallint
Page LRU count
pg_buffercache使用：
查看shared buffers大小：
postgres=# SELECT name,setting,unit,current_setting(name) FROM pg_settings WHERE name='shared_buffers';
name | setting | unit | current_setting
----------------+---------+------+-----------------
shared_buffers | 4096 | 8kB | 32MB
(1 row)
postgres=# select count(*) from pg_buffercache;
count
-------
4096
(1 row)
可见block数量一致，大小一致。
查看当前
数据库buffer的使用情况排名：
SELECT
c.relname,
count(*) AS buffers
FROM pg_class c
INNER JOIN pg_buffercache b
ON b.relfilenode=c.relfilenode
INNER JOIN pg_database d
ON (b.reldatabase=d.oid AND d.datname=current_database())
GROUP BY c.relname
ORDER BY 2 DESC
LIMIT 10;
relname | buffers
---------------------------+---------
pg_statistic | 15
pg_operator | 13
pg_depend_reference_index | 13
pg_depend | 13
pg_rewrite | 8
pg_depend_depender_index | 6
pg_toast_2619 | 6
pg_index | 6
pg_extension | 5
pg_namespace | 5
(10 rows)
使用pg_buffercache比较灵活，可以通过isdirty字段查询脏块，如果是未使用的buffer，那么除了bufferid，其他字段都为空值。
select count(*) from pg_buffercache where isdirty is true；
select count(*)*8/1024||'MB' from pg_buffercache where relfilenode is null and reltablespace is null and reldatabase is null and relforknumber is null and relblocknumber is null and isdirty is null and usagecount is null;
查看buffercache对象的使用大小以及百分比
SELECT
c.relname,
pg_size_pretty(count(*) * 8192) as buffered,
round(100.0 * count(*) /
(SELECT setting FROM pg_settings
WHERE name='shared_buffers')::integer,1)
AS buff