postgresql pg_buffercache

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

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

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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 buffers_percent,

round(100.0 * count(*) * 8192 /

pg_relation_size(c.oid),1)

AS percent_of_relation

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.oid,c.relname

ORDER BY 3 DESC

LIMIT 10;

             relname              | buffered | buffers_percent | percent_of_relation 

----------------------------------+----------+-----------------+---------------------

 pg_statistic                     | 120 kB   |             0.4 |               100.0

 pg_depend                        | 104 kB   |             0.3 |                29.5

 pg_operator                      | 104 kB   |             0.3 |               100.0

 pg_depend_reference_index        | 104 kB   |             0.3 |                50.0

 pg_rewrite                       | 64 kB    |             0.2 |                66.7

 pg_operator_oid_index            | 32 kB    |             0.1 |               100.0

 pg_statistic_relid_att_inh_index | 32 kB    |             0.1 |               100.0

 pg_operator_oprname_l_r_n_index  | 40 kB    |             0.1 |               100.0

 pg_depend_depender_index         | 48 kB    |             0.1 |                22.2

 pg_amop_fam_strat_index          | 32 kB    |             0.1 |               100.0

缓冲区使用分布：

SELECT

c.relname, count(*) AS buffers,usagecount

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,usagecount

ORDER BY c.relname,usagecount;

             relname              | buffers | usagecount 

-----------------------------------+---------+------------

 pg_aggregate                      |       1 |          5

 pg_aggregate_fnoid_index          |       1 |          4

 pg_aggregate_fnoid_index          |       1 |          5

 pg_am                             |       1 |          5

 pg_amop                           |       3 |          5

 pg_amop_fam_strat_index           |       1 |          1

 pg_amop_fam_strat_index           |       3 |          5

 pg_amop_opr_fam_index             |       3 |          5

 pg_amproc                         |       1 |          4

 pg_amproc                         |       1 |          5

 pg_amproc_fam_proc_index          |       2 |          5

 pg_attrdef                        |       1 |          3

 pg_attrdef_adrelid_adnum_index    |       2 |          3

 pg_attrdef_oid_index              |       1 |          1

 pg_attrdef_oid_index              |       1 |          2

 pg_cast                           |       2 |          5

 pg_cast_source_target_index       |       2 |          5

 pg_collation                      |       1 |          1

 pg_collation_oid_index            |       1 |          3

 pg_collation_oid_index            |       2 |          5

 pg_constraint                     |       1 |          1

 pg_default_acl_role_nsp_obj_index |       1 |          5

 pg_depend                         |       3 |          1

 pg_depend                         |       1 |          2

 pg_depend                         |       9 |          5

 pg_depend_depender_index          |       1 |          4

 pg_depend_depender_index          |       5 |          5

 pg_depend_reference_index         |       2 |          1

 pg_depend_reference_index         |       1 |          2

 pg_depend_reference_index         |       1 |          4

 pg_depend_reference_index         |       9 |          5