PostgreSQL_如何查找TOP_SQL_(例如IO消耗最高的SQL)

2017/04/25 17:25
阅读数 8K

标签

PostgreSQL , pg_stat_statements , TOP SQL

背景

数据库是较大型的应用,对于繁忙的数据库,需要消耗大量的内存、CPU、IO、网络资源。

SQL优化是数据库优化的手段之一,优化什么SQL效果最佳呢?首先要了解最耗费资源的SQL,即TOP SQL。

从哪里可以了解数据库的资源都被哪些SQL消耗掉了呢?

资源分为多个维度,CPU,内存,IO等。如何能了解各个维度层面的TOP SQL呢?

pg_stat_statements插件可以用于统计数据库的资源开销,分析TOP SQL。

一、安装pg_stat_statements

pg_stat_statements是PostgreSQL的核心插件之一。可以在编译PostgreSQL时安装,也可以单独安装。

编译时安装

make world  
make install-world  

单独安装

cd src/contrib/pg_stat_statements/  
make; make install  

二、加载pg_stat_statements模块

vi $PGDATA/postgresql.conf  
  
shared_preload_libraries='pg_stat_statements'  

如果要跟踪IO消耗的时间,还需要打开如下参数

track_io_timing = on  

设置单条SQL的最长长度,超过被截断显示(可选)

track_activity_query_size = 2048  

三、配置pg_stat_statements采样参数

vi $PGDATA/postgresql.conf  
  
pg_stat_statements.max = 10000           # 在pg_stat_statements中最多保留多少条统计信息,通过LRU算法,覆盖老的记录。  
pg_stat_statements.track = all           # all - (所有SQL包括函数内嵌套的SQL), top - 直接执行的SQL(函数内的sql不被跟踪), none - (不跟踪)  
pg_stat_statements.track_utility = off   # 是否跟踪非DML语句 (例如DDL,DCL), on表示跟踪, off表示不跟踪  
pg_stat_statements.save = on             # 重启后是否保留统计信息  

重启数据库

pg_ctl restart -m fast  

四、创建pg_stat_statements extension

在需要查询TOP SQL的数据库中,创建extension

create extension pg_stat_statements;  

五、分析TOP SQL

pg_stat_statements输出内容介绍

查询pg_stat_statements视图,可以得到统计信息

SQL语句中的一些过滤条件在pg_stat_statements中会被替换成变量,减少重复显示的问题。

pg_stat_statements视图包含了一些重要的信息,例如:

1. SQL的调用次数,总的耗时,最快执行时间,最慢执行时间,平均执行时间,执行时间的方差(看出抖动),总共扫描或返回或处理了多少行;

2. shared buffer的使用情况,命中,未命中,产生脏块,驱逐脏块。

3. local buffer的使用情况,命中,未命中,产生脏块,驱逐脏块。

4. temp buffer的使用情况,读了多少脏块,驱逐脏块。

5. 数据块的读写时间。

Name Type References Description
userid oid pg_authid.oid OID of user who executed the statement
dbid oid pg_database.oid OID of database in which the statement was executed
queryid bigint - Internal hash code, computed from the statement's parse tree
query text - Text of a representative statement
calls bigint - Number of times executed
total_time double precision - Total time spent in the statement, in milliseconds
min_time double precision - Minimum time spent in the statement, in milliseconds
max_time double precision - Maximum time spent in the statement, in milliseconds
mean_time double precision - Mean time spent in the statement, in milliseconds
stddev_time double precision - Population standard deviation of time spent in the statement, in milliseconds
rows bigint - Total number of rows retrieved or affected by the statement
shared_blks_hit bigint - Total number of shared block cache hits by the statement
shared_blks_read bigint - Total number of shared blocks read by the statement
shared_blks_dirtied bigint - Total number of shared blocks dirtied by the statement
shared_blks_written bigint - Total number of shared blocks written by the statement
local_blks_hit bigint - Total number of local block cache hits by the statement
local_blks_read bigint - Total number of local blocks read by the statement
local_blks_dirtied bigint - Total number of local blocks dirtied by the statement
local_blks_written bigint - Total number of local blocks written by the statement
temp_blks_read bigint - Total number of temp blocks read by the statement
temp_blks_written bigint - Total number of temp blocks written by the statement
blk_read_time double precision - Total time the statement spent reading blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)
blk_write_time double precision - Total time the statement spent writing blocks, in milliseconds (if track_io_timing is enabled, otherwise zero)

最耗IO SQL

单次调用最耗IO SQL TOP 5

select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time)/calls desc limit 5;  

总最耗IO SQL TOP 5

select userid::regrole, dbid, query from pg_stat_statements order by (blk_read_time+blk_write_time) desc limit 5;  

最耗时 SQL

单次调用最耗时 SQL TOP 5

select userid::regrole, dbid, query from pg_stat_statements order by mean_time desc limit 5;  

总最耗时 SQL TOP 5

select userid::regrole, dbid, query from pg_stat_statements order by total_time desc limit 5;  

响应时间抖动最严重 SQL

select userid::regrole, dbid, query from pg_stat_statements order by stddev_time desc limit 5;  

最耗共享内存 SQL

select userid::regrole, dbid, query from pg_stat_statements order by (shared_blks_hit+shared_blks_dirtied) desc limit 5;  

最耗临时空间 SQL

select userid::regrole, dbid, query from pg_stat_statements order by temp_blks_written desc limit 5;  

六、重置统计信息

pg_stat_statements是累积的统计,如果要查看某个时间段的统计,需要打快照,建议参考

《PostgreSQL AWR报告(for 阿里云ApsaraDB PgSQL)》

用户也可以定期清理历史的统计信息,通过调用如下SQL

select pg_stat_statements_reset();  
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