drop table if exists t;
select @@optimizer_switch;
@@optimizer_switch
index_merge=on,index_merge_union=on,index_merge_sort_union=on,index_merge_intersection=on,index_merge_sort_intersection=off,engine_condition_pushdown=off,index_condition_pushdown=on,derived_merge=on,derived_with_keys=on,firstmatch=on,loosescan=on,materialization=on,in_to_exists=on,semijoin=on,partial_match_rowid_merge=on,partial_match_table_scan=on,subquery_cache=on,mrr=off,mrr_cost_based=off,mrr_sort_keys=off,outer_join_with_cache=on,semijoin_with_cache=on,join_cache_incremental=on,join_cache_hashed=on,join_cache_bka=on,optimize_join_buffer_size=off,table_elimination=on,extended_keys=on,exists_to_in=on,orderby_uses_equalities=on,condition_pushdown_for_derived=on,split_materialized=on,condition_pushdown_for_subquery=on
create table t (a int not null, b int not null, c int not null, d int not null, primary key(a,b), key(c,a)) engine=innodb;
insert into t values (0,0,0,0),(0,1,0,1);
explain select c,a,b from t where c=0 and a=0 and b=1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t	const	PRIMARY,c	PRIMARY	8	const,const	1	
flush status;
select c,a,b from t where c=0 and a=0 and b=1;
c	a	b
0	0	1
show status like 'handler_read%';
Variable_name	Value
Handler_read_first	0
Handler_read_key	1
Handler_read_last	0
Handler_read_next	0
Handler_read_prev	0
Handler_read_retry	0
Handler_read_rnd	0
Handler_read_rnd_deleted	0
Handler_read_rnd_next	0
explain select c,a,b from t force index (c) where c=0 and a=0 and b=1;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t	const	c	c	12	const,const,const	1	Using index
flush status;
select c,a,b from t force index (c) where c=0 and a=0 and b=1;
c	a	b
0	0	1
show status like 'handler_read%';
Variable_name	Value
Handler_read_first	0
Handler_read_key	1
Handler_read_last	0
Handler_read_next	0
Handler_read_prev	0
Handler_read_retry	0
Handler_read_rnd	0
Handler_read_rnd_deleted	0
Handler_read_rnd_next	0
drop table t;