SQL优化之PostgreSQL的pg_stat_statements统计模块

 pg_stat_statements模块含义？

• pg_stat_statements模块提供一种方法追踪一个服务器所执行的所有 SQL 语句的执行统计信息，可以用于统计数据库的资源开销，分析TOP SQL。
• 如下所示表中所代表的字段：f放到

-- SQL统计
SELECT 
userid AS 执行者id ,
dbid AS 执行数据库id ,
query AS 执行的语句 ,
calls AS 执行次数 ,
total_time AS 执行总时间,
total_time/calls AS 执行平均时间,
rows AS 影响的总行数,
min_time,
max_time,
mean_time AS 平均时间
FROM pg_stat_statements

SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;

• 各个字段代表的含义信息？

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;

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;

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();

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;

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

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