1.如果COUNT(列）中列有索引時且列為空時
COUNT(列）比COUNT(*)要快，因為count(*)不可以使用索引
2.如果COUNT(列）中列有索引時且列不為空時
count(列)和count(*)其實一樣快，如果索引列是非空的，count(*)可用到索引
但是當count(列)中的列在表中的位置越靠后速度越慢
select u.* from t_user u where u.empe_code in ('0000126991','1111');--ok
select u.* from t_user u where u.dept_code='0003130122'; -- 2 no
select u.* from t_user u where u.job_code='P100021404003'; --3 no
select u.* from t_user u where u.empe_code='0000126991' and u.job_code='P100021404003';-- 13 ok
select u.* from t_user u where u.job_code='P100021404003' and u.empe_code='0000126991'; --31 ok
select u.* from t_user u where u.job_code='P100021404003' and u.dept_code='0000126991'; --32 no
select u.* from t_user u where u.dept_code='0000126991' and u.job_code='P100021404003'; --23 no
select u.* from t_user u where u.empe_code='0000126991' and u.dept_code='0000126991' and u.job_code='P100021404003'; --123 ok
select u.* from t_user u where u.empe_code='0000126991' and u.job_code='P100021404003' and u.dept_code='0000126991';--132 ok
select u.* from t_user u where u.job_code='P100021404003' and u.dept_code='0000126991' and u.empe_code='0000126991';--321 ok
create index user_empe_dept_job on t_user (empe_code,dept_code,job_code);
創建多列索引后，只要索引的首個字段出現就會被使用，與出現順序無關