MySql查詢計划選擇要檢查的行較少的索引，但執行速度慢得多

Question

我有一個簡單的查詢-它只查看一個表，並且有2列可供選擇以建立索引，一個ErrorTypeId（指向約20個唯一值的查找表）和DateOccurred（一個datetime列，其中可以有任何日期） 。

在大多數情況下，這很好，但是在某些情況下查詢將超時。 對其進行調查，所選擇的索引（ErrorTypeId）在計划中的行比其他索引（DateOccurred）少，但是由於某種原因，與我手動強制它使用其他索引相比，它需要更長的運行時間。

/* 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;

當有更多行要查看時，為什么第二個查詢更快？

有沒有一種方法可以幫助MySql在不使用USE / FORCE索引的情況下選擇更快的索引？

Answer 1

查詢緩慢的原因是索引編制效率低下。

在(ErrorTypeId, DateOccurred)上添加一個復合索引，您將不需要任何強制或提示。