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（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。
| 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 | 无 | Internal hash code, computed from the statement’s parse tree. |
| query | text | 无 | Text of a representative statement. |
| calls | bigint | 无 | Number of times executed. |
| total_time | double precision | 无 | Total time spent in the statement, in milliseconds. |
| min_time | double precision | 无 | Minimum time spent in the statement, in milliseconds. |
| max_time | double precision | 无 | Maximum time spent in the statement, in milliseconds. |
| mean_time | double precision | 无 | Mean time spent in the statement, in milliseconds. |
| stddev_time | double precision | 无 | Population standard deviation of time spent in the statement, in milliseconds. |
| rows | bigint | 无 | Total number of rows retrieved or affected by the statement. |
| shared_blks_hit | bigint | 无 | Total number of shared block cache hits by the statement. |
| shared_blks_read | bigint | 无 | Total number of shared blocks read by the statement. |
| shared_blks_dirtied | bigint | 无 | Total number of shared blocks dirtied by the statement. |
| shared_blks_written | bigint | 无 | Total number of shared blocks written by the statement. |
| local_blks_hit | bigint | 无 | Total number of local block cache hits by the statement. |
| local_blks_read | bigint | 无 | Total number of local blocks read by the statement. |
| local_blks_dirtied | bigint | 无 | Total number of local blocks dirtied by the statement. |
| local_blks_written | bigint | 无 | Total number of local blocks written by the statement. |
| temp_blks_read | bigint | 无 | Total number of temp blocks read by the statement. |
| temp_blks_written | bigint | 无 | Total number of temp blocks written by the statement. |
| blk_read_time | double precision | 无 | Total time the statement spent reading blocks, in milliseconds (if track_io_timing is enabled, otherwise zero). |
| blk_write_time | double precision | 无 | 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();
