子查询到底走不走索引？

CREATE TABLE zichaxuntest (
a int(11) NOT NULL,
b varchar(255) DEFAULT NULL,
c varchar(255) DEFAULT NULL,
d varchar(255) DEFAULT NULL,
PRIMARY KEY (a),
KEY bIndex (b) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

实验1

SELECT a,b,c from (SELECT a,b,c FROM zichaxunTest) gg where gg.b =1;

EXPLAIN SELECT a,b,c from (SELECT a,b,c FROM zichaxunTest) gg where gg.a =1;

实验2

EXPLAIN SELECT a,b,c from (SELECT a,b,c FROM zichaxunTest where b =2 ) gg where gg.b =1;

EXPLAIN SELECT a,b,c from (SELECT a,b,c FROM zichaxunTest where b =2) gg where gg.a =1;

总结： 5.7.21-log 版本 ， windows ,
外层有主键索引的话，走索引，否则不走