性能分析
1. explain 查看sql执行计划，得出索引使用情况等信息
2. show profiling 查看sql所有执行步骤以及用时，使用步骤如下
1）开启性能剖析
mysql> set profiling = 1;
2）执行你要查询的sql
3）查看性能剖析接收到的sql以及耗时
mysql> show profiles;
4）查看sql执行的详细步骤耗时(这里query后的id是步骤3中的QUERY_ID)
mysql> show prifile for query 1;
5）这样可能可阅读性不强，不容易观察出耗时最多的步骤以及执行次数等
mysql> set @query_id = 1;
mysql>
SELECT state, SUM(duration) AS total_r, round(100 * SUM(duration) / (
SELECT SUM(duration)
FROM information_schema.profiling
WHERE QUERY_ID = @query_id
), 2) AS pct_r, COUNT(*) AS calls, SUM(duration) / COUNT(*) AS ffp
FROM information_schema.profiling
WHERE QUERY_ID = @query_id
GROUP BY state
ORDER BY total_r DESC;
这样就可以清晰看到sql语句耗时在哪些步骤