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移除书签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。
create extension pg_stat_statements;
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。
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。
select pg_stat_statements_reset();
