我正在尝试估算以下查询可能花费的时间：
mysql> EXPLAIN SELECT t1.col1, t1_col4 FROM t1 LEFT JOIN t2 ON t1.col1=t2.col1 WHERE col2=0 AND col3 IS NULL; +----+-------------+--------------------+------+---------------+------------+---------+-----------------------------+---------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------------------+------+---------------+------------+---------+-----------------------------+---------+--------------------------+ | 1 | SIMPLE | t1 | ref | foobar | foobar | 4 | const | 9715129 | | | 1 | SIMPLE | t2 | ref | col1 | col1 | 4 | db2.t1.col1 | 42318 | Using where; Using index | +----+-------------+--------------------+------+---------------+------------+---------+-----------------------------+---------+--------------------------+ 2 rows in set (0.00 sec) mysql>
使用SHOW PROFILES语法可以做到这一点。打开MySQL会话时，可以将变量“分析”设置为1或ON。
mysql> SET profiling = 1;
因此，所有发送到服务器的语句将被分析并存储在历史记录中，并在以后通过键入以下命令显示：
mysql> SHOW PROFILES;
从MySQL手册中可以看到：
mysql> SET profiling = 1; Query OK, 0 rows affected (0.00 sec) mysql> DROP TABLE IF EXISTS t1; Query OK, 0 rows affected, 1 warning (0.00 sec) mysql> CREATE TABLE T1 (id INT); Query OK, 0 rows affected (0.01 sec) mysql> SHOW PROFILES; +----------+----------+--------------------------+ | Query_ID | Duration | Query | +----------+----------+--------------------------+ | 0 | 0.000088 | SET PROFILING = 1 | | 1 | 0.000136 | DROP TABLE IF EXISTS t1 | | 2 | 0.011947 | CREATE TABLE t1 (id INT) | +----------+----------+--------------------------+ 3 rows in set (0.00 sec) mysql> SHOW PROFILE; +----------------------+----------+ | Status | Duration | +----------------------+----------+ | checking permissions | 0.000040 | | creating table | 0.000056 | | After create | 0.011363 | | query end | 0.000375 | | freeing items | 0.000089 | | logging slow query | 0.000019 | | cleaning up | 0.000005 | +----------------------+----------+ 7 rows in set (0.00 sec) mysql> SHOW PROFILE FOR QUERY 1; +--------------------+----------+ | Status | Duration | +--------------------+----------+ | query end | 0.000107 | | freeing items | 0.000008 | | logging slow query | 0.000015 | | cleaning up | 0.000006 | +--------------------+----------+ 4 rows in set (0.00 sec) mysql> SHOW PROFILE CPU FOR QUERY 2; +----------------------+----------+----------+------------+ | Status | Duration | CPU_user | CPU_system | +----------------------+----------+----------+------------+ | checking permissions | 0.000040 | 0.000038 | 0.000002 | | creating table | 0.000056 | 0.000028 | 0.000028 | | After create | 0.011363 | 0.000217 | 0.001571 | | query end | 0.000375 | 0.000013 | 0.000028 | | freeing items | 0.000089 | 0.000010 | 0.000014 | | logging slow query | 0.000019 | 0.000009 | 0.000010 | | cleaning up | 0.000005 | 0.000003 | 0.000002 | +----------------------+----------+----------+------------+
参考文献（在更新日期：2014年9月4日）： - SHOW PROFILE语法 - INFORMATION_SCHEMA信息PROFILING表 - 如何使用MySQL查询剖析（ 数字海洋最近公布的关于此问题的大文章。 ）
