原始 SQL（执行工夫 80s+）
SELECT count(id) as id FROM task WHERE cid=100015 AND sid IN(112310,112316,106959,110780,112324,112331,112317) AND flag='9'
优化后 SQL（均匀执行工夫 100ms）
select sum(id) as id from (SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=112310 AND flag=9
union all
SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=112316 AND flag=9
union all
SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=106959 AND flag=9
union all
SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=110780 AND flag=9
union all
SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=112324 AND flag=9
union all
SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=112331 AND flag=9
union all
SELECT count(*) as id FROM task WHERE AND cid=123456 AND sid=112317 AND flag=9
) t
阐明：
mysql 版本 5.6
表总的数据量 60 万 +
cid 和 flag 为 int 类型，参数去掉引号，防止隐式转换
(cid sid flag) 是组合索引，应用 uinon all 拆分后，遍历的表多了，应用组全但更高效