使用pg_buffercache查看缓存区缓存

PG提供了一个扩展pg_buffercache来查看缓存区的内容。

create database test;
CREATE DATABASE

create extension pg_buffercache ;
CREATE EXTENSION

创建个测试数据库test,并且添加扩展。

psql -d test

test=# select distinct reldatabase from pg_buffercache ;
 reldatabase 
-------------
            
       16394
       13322
           0
(4 rows)

 在缓存区中找到两个数据库的内容，带0的记录表示缓存区未使用。

test=# ! oid2name
All databases:
    Oid  Database Name  Tablespace
----------------------------------
  13322       postgres  pg_default
  13321      template0  pg_default
      1      template1  pg_default
  16394           test  pg_default

 通过SQL更直观的来看一下：

select 
c.relname,
count(*) as buffers
from pg_class c
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;

              relname              | buffers 
-----------------------------------+---------
 pg_operator                       |      14
 pg_depend_reference_index         |      12
 pg_depend                         |      10
 pg_rewrite                        |       6
 pg_description                    |       6
 pg_amop                           |       5

 这些内容，都是数据字典视图。

test=# create table lsang(id serial,name varchar(20));
CREATE TABLE

test=# insert into lsang(name) values('Michael');
INSERT 0 1

test=# select * from lsang;
 id |  name   
----+---------
  1 | Michael
(1 row)


select 
c.relname,
count(*) as buffers
from pg_class c
join pg_buffercache b
on b.relfilenode = c.relfilenode
inner join pg_database d
on (b.reldatabase = d.oid and d.datname = current_database())
where c.relname not like 'pg%' 
group by c.relname
order by 2 desc;

   relname    | buffers 
--------------+---------
 lsang        |       1
 lsang_id_seq |       1
(2 rows)

 创建表并插入数据，我们通过pg_buffercache能够查询到buffers。

我们来看看数据缓存区是否为脏的。

select 
c.relname,
b.isdirty
from pg_class c
join pg_buffercache b
on b.relfilenode = c.relfilenode
inner join pg_database d
on (b.reldatabase = d.oid and d.datname = current_database())
where c.relname not like 'pg%' ;
   relname    | isdirty 
--------------+---------
 lsang_id_seq | f
 lsang        | f

注意，isdirty标记的为f:

test=# update lsang  set name = 'Michael.Sang';
UPDATE 1

再次查询结果:
   relname    | isdirty 
--------------+---------
 lsang_id_seq | f
 lsang        | t
(2 rows)

结果告诉我们，缓存区是脏的，我们可以强制设置个检查点:

test=# checkpoint ;
CHECKPOINT

 重复上面查询:

   relname    | isdirty 
--------------+---------
 lsang_id_seq | f
 lsang        | f
(2 rows)

 这样缓存区就不再是脏的了。