查找最耗费资源的 SQL（Top SQL）

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，而为了达到 SQL 优化的最佳效果，您首先需要了解最消耗资源的 SQL（Top SQL），例如 IO 消耗最高的 SQL。
数据库资源分为多个维度、CPU、内存、IO 等，为能够从各个维度层面查找最消耗数据库资源的 SQL，您可以使用 pg_stat_statements 插件统计数据库的资源开销和分析 Top SQL。
本文将通过示例介绍如何创建 pg_stat_statements 插件、如何分析 Top SQL 以及如何重置统计信息。

创建 pg_stat_statements 插件


执行如下命令，在需要查询 TOP SQL 的数据库中，创建 pg_stat_statements 插件。

1. [backcolor=transparent]create extension pg_stat_statements[backcolor=transparent];

分析 TOP SQL



pg_stat_statements 输出内容介绍


通过查询 pg_stat_statements 视图，您可以得到数据库资源开销的统计信息。SQL 语句中的一些过滤条件在 pg_stat_statements 中会被替换成变量，可以减少重复显示的问题。
pg_stat_statements 视图包含了一些重要信息，例如：

• 
  SQL 的调用次数，总耗时，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、返回或处理了多少行。
• 
  shared buffer 的使用情况：命中、未命中、产生脏块、驱逐脏块。
• 
  local buffer 的使用情况：命中、未命中、产生脏块、驱逐脏块。
• 
  temp buffer 的使用情况：读了多少脏块、驱逐脏块。
• 
  数据块的读写时间。

下表列出了 pg_stat_statements 输出内容中各参数的含义。

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


执行如下命令，查询单次调用最耗 IO SQL TOP 5。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] [backcolor=transparent]([backcolor=transparent]blk_read_time[backcolor=transparent]+[backcolor=transparent]blk_write_time[backcolor=transparent])/[backcolor=transparent]calls desc limit [backcolor=transparent]5[backcolor=transparent];

执行如下命令，查询总最耗 IO SQL TOP 5。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] [backcolor=transparent]([backcolor=transparent]blk_read_time[backcolor=transparent]+[backcolor=transparent]blk_write_time[backcolor=transparent])[backcolor=transparent] desc limit [backcolor=transparent]5[backcolor=transparent];

最耗时 SQL


执行如下命令，查询单次调用最耗时 SQL TOP 5。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] mean_time desc limit [backcolor=transparent]5[backcolor=transparent];

执行如下命令，查询总最耗时 SQL TOP 5。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] total_time desc limit [backcolor=transparent]5[backcolor=transparent];

响应时间抖动最严重 SQL


执行如下命令，查询响应时间抖动最严重 SQL。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] stddev_time desc limit [backcolor=transparent]5[backcolor=transparent];

最耗共享内存 SQL


执行如下命令，查询最耗共享内存 SQL。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] [backcolor=transparent]([backcolor=transparent]shared_blks_hit[backcolor=transparent]+[backcolor=transparent]shared_blks_dirtied[backcolor=transparent])[backcolor=transparent] desc limit [backcolor=transparent]5[backcolor=transparent];

最耗临时空间 SQL


执行如下命令，查询最耗临时空间 SQL。

1. [backcolor=transparent]select[backcolor=transparent] userid[backcolor=transparent]::[backcolor=transparent]regrole[backcolor=transparent],[backcolor=transparent] dbid[backcolor=transparent],[backcolor=transparent] query [backcolor=transparent]from[backcolor=transparent] pg_stat_statements order [backcolor=transparent]by[backcolor=transparent] temp_blks_written desc limit [backcolor=transparent]5[backcolor=transparent];

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，建议您参见文档 《PostgreSQL AWR报告(for 阿里云ApsaraDB PgSQL)》。
您也可以通过执行如下命令，来定期清理历史统计信息。

1. [backcolor=transparent]select[backcolor=transparent] pg_stat_statements_reset[backcolor=transparent]();