mysql索引失效情况总结(未完)

-- 对于所有本应该走索引的，当mysql发现通过索引扫描的行记录数超过全表的10%-30%时，优化器可能会放弃走索引，自动变成全表扫描
-- 1. =走索引，!=、也走索引
explain select id2 from user where varchar_short4  = '0';            						--  = 范围小，肯定走索引。ref。key_len=11
explain select id2 from user where varchar_short4  = '0000';										--  = 范围大，肯定走索引。ref
explain select id2 from user where varchar_short4 != '0000';										-- != 范围小，    走索引。range
explain select id2 from user where varchar_short4 != '0';												-- != 范围大，  不走索引。
-- 2. 、>=、 '1';																													--   走索引,key_len=11
explain select id2 from user2 where varchar_short7 > '1'        and varchar_short8 = '0' and varchar_short9 = '0';-服务器托管网-   走索引,key_len=33
explain select id2 from user2 where varchar_short7 like '1%'																											--   走索引,key_len=11
explain select id2 from user2 where varchar_short7 like '1%' and varchar_short8 = '0' and varchar_short9 = '0';		--   走索引,key_len=33
explain select id2 from user2 where varchar_short7 = '0' or  varchar_short8 = '0';																-- 不走索引

explain select id2 from user ;

explain select * from user where bigint2 = 1 limit 1;
explain select distinct id2 from user where id2  > '1';
explain select id2 from user limit 100,1;

select * from user;

explain select id2 from user where id2   = '1';            										--  =范围小，肯定走索引服务器托管网。const
explain select id2 from user where id2  != '1';            										--  =范围小，肯定走索引。const