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查找最耗费资源的 SQL(Top SQL)

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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,包括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。

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。

CREATE EXTENSION pg_stat_statements;

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(反映抖动),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。

参数名称

类型

参考

说明

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).

分析 TOP SQL

  • 最耗 IO SQL

    • 执行如下命令,查询单次调用最耗 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;
    • 执行如下命令,查询总最耗 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

    • 执行如下命令,查询单次调用最耗时 SQL TOP 5。

      SELECT userid::regrole, dbid, query FROM pg_stat_statements ORDER BY mean_time DESC LIMIT 5;
    • 执行如下命令,查询总最耗时 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

    执行如下命令,查询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

    执行如下命令,查询最耗共享内存 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

    执行如下命令,查询最耗临时空间 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,需要查询快照的信息,详情请参见《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。

SELECT pg_stat_statements_reset();

参考文档

PostgreSQL 9.6.2 Documentation — F.29. pg_stat_statements

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