4.2百万条数据索引字段为 myfield char(32)查询测试语句:select count(distinct myfield) as cnt from mytab;索引前:1 row in set (45.88 sec)索引后:1 row in set (1 min 7.82 sec)何解?
4.2百万条数据索引字段为 myfield char(32)查询测试语句:select count(distinct myfield) as cnt from mytab;索引前:1 row in set (45.88 sec)索引后:1 row in set (1 min 7.82 sec)何解?
索引本身就是排序的
show index from mytab;
explain select count(distinct myfield) as cnt from mytab;这样大家可以基于实情进行讨论。