我有一个简单的查询 - 它只查看一个表,并可以选择 2 列用于索引、ErrorTypeId(指向大约 20 个唯一值的查找表)和 DateOccurred(可以包含任何日期的日期时间列)在里面)。
在大多数情况下都很好,但在某些情况下查询会超时。仔细观察,所选索引 (ErrorTypeId) 的计划中的行数比另一个索引 (DateOccurred) 少,但由于某种原因,运行时间比我手动强制它使用其他索引要长得多。
/* This execution plan has fewer rows than the second one - and as a result this is
* the one chosen. But it is much much slower.
*/
EXPLAIN
SELECT * FROM my_log_table
WHERE DateOccurred BETWEEN '2013-06-11 00:00:00' AND '2013-06-22 00:00:00' AND ErrorTypeId = 4
ORDER BY DateOccurred DESC
LIMIT 1000;
/* Many more rows - but much much quicker to execute.
* Is it the spread of data across pages? Is there a
* way other than USE/FORCE INDEX to improve execution plan?
*/
EXPLAIN
SELECT * FROM my_log_table USE INDEX (DateOccurred_Index)
WHERE DateOccurred BETWEEN '2013-06-11 00:00:00' AND '2013-06-22 00:00:00' AND ErrorTypeId = 4
ORDER BY DateOccurred DESC
LIMIT 1000;
为什么当有更多行需要查看时,第二个查询会更快?
有没有办法帮助MySql选择更快的索引而不使用USE/FORCE索引?
最佳答案
查询速度慢的原因是索引效率低下。
在(ErrorTypeId, DateOccurred)
上添加复合索引,您将不需要任何强制或提示。
关于MySql查询计划选择行数较少的索引来检查,但执行速度慢得多,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/17298772/