FORCE INDEX 通常用来对查询强制使用一个或者多个索引。 MySQL 通常会根据统计信息选择正确的索引,但是当查询优化器选择了错误的索引或者根本没有使用索引的时候,这个提示将非常有用。
IGNORE INDEX 提示会禁止查询优化器使用指定的索引。在具有多个索引的查询时,可以用来指定不需要优化器使用的那个索引,还可以在删除不必要的索引之前在查询中禁止使用该索引。
FORCE INDEX/IGNORE INDEX 的语法:
SELECT *** FROM TABLE [{USE|IGNORE|FORCE} INDEX (key_list)] WHERE ……
下面的例子是使用 IGNORE INDEX 以后,执行计划的变化情况,默认的执行计划是按照主键索引进行扫描,如果我们使用 IGNORE INDEX 忽略主键索引,则会按照全表扫描执行
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