MySQL index hint

语法：

tbl_name [[AS] alias] [index_hint_list]

index_hint_list:
index_hint [, index_hint] ...

index_hint:
USE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
| IGNORE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
| FORCE {INDEX|KEY}
[FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

index_list:
index_name [, index_name] ...

官方说明：

The USE INDEX (index_list) hint tells MySQL to use only one of the named indexes to find rows in the table. The alternative syntax IGNORE INDEX

(index_list) tells MySQL to not use some particular index or indexes. These hints are useful if EXPLAIN shows that MySQL is using the wrong

index from the list of possible indexes.

The FORCE INDEX hint acts like USE INDEX (index_list), with the addition that a table scan is assumed to be very expensive. In other words, a table

scan is used only if there is no way to use one of the named indexes to find rows in the table.

Each hint requires the names of indexes, not the names of columns. To refer to a primary key, use the name PRIMARY. To see the index names for

a table, use SHOW INDEX.

An index_name value need not be a full index name. It can be an unambiguous prefix of an index name. If a prefix is ambiguous, an error occurs.

USE INDEX（ 这个可以限制索引的使用范围）

有时我们在数据表里建立了很多索引，当MySQL对索引进行选择时，这些索引都在考虑的范围内。但有时我们希望MySQL只考虑几个索引，而不是全部的索引，这就需要用到USE INDEX对查询语句进行设置

IGNORE INDEX（限制不使用索引的范围）

和上面的use index刚好是相反的。

FORCE INDEX(强迫使用某一个索引)

上面的两个例子都是给MySQL提供一个选择范围。而有时我们希望MySQL必须要使用某一个索引(由于 MySQL在查询时只能使用一个索引，因此只能强迫MySQL使用一个索引)。这就需要使用FORCE INDEX来完成这个功能。

更详细的信息可以查看mysql官方文档