我有一个简单的查询 - 它只查看一个表，并且可以选择2列进行索引，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datetime列）在里面）。
在大多数情况下它很好但在某些情况下查询会超时。调查一下，所选择的索引（ErrorTypeId）在其计划中的行数少于另一个（DateOccurred），但由于某种原因，运行时间比手动强制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;
当有更多行要查看时，为什么第二个查询会更快？
有没有办法帮助MySql选择更快的索引而不使用USE / FORCE索引？
答案 0 :(得分：0)
查询速度慢的原因是索引效率低下。
在(ErrorTypeId, DateOccurred)上添加综合索引，您不需要任何强制或提示。