mysql 的 sql 性能分析器主要用途是显示 sql 执行的整个过程中各项资源的使用情况。分析器可以更好的展示出不良 SQL 的性能问题所在。
下面我们举例介绍一下 MySQL SQL Profiler 的使用方法：
首先，开启 MySQL SQL Profiler
| mysql> SELECT @@profiling; +-------------+ | @@profiling | +-------------+ | 0 | +-------------+ 1 row in set (0.00 sec) mysql> SET profiling = 1; Query OK, 0 rows affected (0.00 sec) mysql> SELECT @@profiling; +-------------+ | @@profiling | +-------------+ | 1 | +-------------+ 1 row in set (0.00 sec) |
默认情况下 profiling 的值为 0 表示 MySQL SQL Profiler 处于 OFF 状态，开启 SQL 性能分析器后 profiling 的值为 1.
通过 sql 性能分析器，我们来对比一下 下列语句前后 2 次执行过程的差异，对我们了解 sql 的详细执行过程是非常有帮助的。
| mysql> create table t_engines select * from t_engines1; Query OK, 57344 rows affected (0.10 sec) Records: 57344 Duplicates: 0 Warnings: 0 mysql> select count(*) from t_engines; +----------+ | count(*) | +----------+ | 57344 | +----------+ 1 row in set (0.00 sec) mysql> select count(*) from t_engines; +----------+ | count(*) | +----------+ | 57344 | +----------+ 1 row in set (0.00 sec) mysql> SHOW PROFILES; +----------+------------+-------------------------------------------------+ | Query_ID | Duration | Query | +----------+------------+-------------------------------------------------+ | 26 | 0.10213775 | create table t_engines select * from t_engines1 | | 27 | 0.00032775 | select count(*) from t_engines | | 28 | 0.00003850 | select count(*) from t_engines | +----------+------------+-------------------------------------------------+ 15 rows in set (0.01 sec) mysql> SHOW PROFILE FOR QUERY 27; +--------------------------------+------------+ | Status | Duration | +--------------------------------+------------+ | (initialization) | 0.00000425 | | checking query cache for query | 0.00004050 | | checking permissions | 0.00001050 | | Opening tables | 0.00018250 | | System lock | 0.00000450 | | Table lock | 0.00001775 | | init | 0.00001075 | | optimizing | 0.00000550 | | executing | 0.00002775 | | end | 0.00000450 | | query end | 0.00000325 | | storing result in query cache | 0.00000400 | | freeing items | 0.00000400 | | closing tables | 0.00000500 | | logging slow query | 0.00000300 | +--------------------------------+------------+ 15 rows in set (0.00 sec) mysql> SHOW PROFILE FOR QUERY 28; +-------------------------------------+------------+ | Status | Duration | +-------------------------------------+------------+ | (initialization) | 0.00000350 | | checking query cache for query | 0.00000750 | | checking privileges on cached query | 0.00000500 | | checking permissions | 0.00000525 | | sending cached result to client | 0.00001275 | | logging slow query | 0.00000450 | +-------------------------------------+------------+ 6 rows in set (0.00 sec) mysql> SELECT sum( FORMAT(DURATION, 6)) AS DURATION FROM INFORMATION_SCHEMA.PROFILING WHERE QUERY_ID =27 ORDER BY SEQ; +----------+ | DURATION | +----------+ | 0.000326 | +----------+ 1 row in set (0.00 sec) mysql> SELECT sum( FORMAT(DURATION, 6)) AS DURATION FROM INFORMATION_SCHEMA.PROFILING WHERE QUERY_ID =28 ORDER BY SEQ; +----------+ | DURATION | +----------+ | 0.000039 | +----------+ 1 row in set (0.00 sec) mysql> |
从上面的例子中我们可以清晰的看出 2 次执行 count 语句的差别， SHOW PROFILE FOR QUERY 27 展现的是第一次 count 统计的执行过程，包含了 Opening tables 、 Table lock 等操作 。而 SHOW PROFILE FOR QUERY 28 展示了第二次 count 统计的执行过程 , 第二次 count 直接从查询缓存中返回 count 统计结果，通过对比 2 次统计的总执行时间发现，缓存读的速度接近物理读的 10 倍。通过使用 SQL 性能分析器可以帮助我们对一些比较难以确定性能问题的 SQL 进行诊断，找出问题根源。
