數據庫索引失效的場景

create table t1 (
    a int PRIMARY key,
    b int,
    c int,
    d int,
    e varchar(20)
)ENGINE=INNODB

insert into t1 value(4,3,1,1,'d');
insert into t1 value(1,1,1,1,'a');
insert into t1 value(8,8,8,8,'h');
insert into t1 value(2,2,2,2,'b');
insert into t1 value(5,2,3,5,'e');
insert into t1 value(3,3,3,3,'c');
insert into t1 value(7,4,5,5,'g');
insert into t1 value(6,6,4,4,'f');

-- 建立索引
create index idx_t1_bcd on t1(b,c,d);　　　　# 組合索引
create index idx_t1_e on t1(e);　　　　　　　 # 普通索引

-- 查看索引結構
show index from t1;