mysql - MySql 查询计划选择要检查的行数较少的索引，但执行速度要慢得多

Question

我有一个简单的查询 - 它只查看一个表，并且可以选择 2 个用于索引的列，一个 ErrorTypeId（指向一个包含大约 20 个唯一值的查找表）和 DateOccurred（一个可以包含任何日期的日期时间列） .

在大多数情况下这很好，但在某些情况下查询会超时。调查一下，选择的索引 (ErrorTypeId) 在其计划中的行数比另一个 (DateOccurred) 少，但由于某种原因，运行时间比我手动强制它使用另一个索引要长得多。

/* 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，为什么第二个查询更快？

有没有办法在不使用 USE/FORCE 索引的情况下帮助 MySql 选择更快的索引？

Answer 1

查询速度慢的原因是索引效率低下。

添加一个复合索引，(ErrorTypeId, DateOccurred)您将不需要任何强制或提示。