条件查询加了条件后 执行速度变得很慢

Question

select   td.name tname,count(O.NAME) sumname,0 jr,0 nbj
from T_QL_CASE q,T_SYS_ORGAN O,T_SYS_DISTRICT TD,T_SX_SERVICE SX
where q.DEL_FLAG='N' and  q.service_oid=sx.service_oid and sx.implement_organ_oid =O.OID and TD.OID = O.DISTRICT_OID 
and sx.service_type_oid in ('402881fd5fb4a7ed015fb4ac939a0002','402881ef5b8a0db1015b8a1286620001',
'402881ef5b8a0db1015b8a1323250003','402881fd5f952f81015f9af6c9190044','402881ef5b8a0db1015b8a13d8b50004','402881fd5fb4a7ed015fb4ad6feb0003')
GROUP by td.name,O.NAME

语句加了 q.DEL_FLAG='N' 后执行速度变得很慢，不加执行是2s 加了就变 9s ,这张表的 DEL_FLAG也加了索引，执行计划查看 如图

为啥去掉一个有索引的条件后 执行反而变快，求教

Answer 1

这种删除标识的字段不需要加索引，字段内容单一，查询优化可能会全表扫描，有时候走了索引说不定会比不走索引花费更多时间，MySQL的回表也要花时间