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drop table if exists t1;
create table t1 (id int not null, str char(10), unique(str));
insert into t1 values (1, null),(2, null),(3, "foo"),(4, "bar");
select * from t1 where str is null;
id str
1 NULL
2 NULL
select * from t1 where str="foo";
id str
3 foo
explain select * from t1 where str is null;
table type possible_keys key key_len ref rows Extra
t1 ref str str 11 const 1 where used
explain select * from t1 where str="foo";
table type possible_keys key key_len ref rows Extra
t1 const str str 11 const 1
explain select * from t1 ignore key (str) where str="foo";
table type possible_keys key key_len ref rows Extra
t1 ALL NULL NULL NULL NULL 4 where used
explain select * from t1 use key (str,str) where str="foo";
table type possible_keys key key_len ref rows Extra
t1 const str str 11 const 1
explain select * from t1 use key (str,str,foo) where str="foo";
Key column 'foo' doesn't exist in table
explain select * from t1 ignore key (str,str,foo) where str="foo";
Key column 'foo' doesn't exist in table
drop table t1;
explain select 1;
Comment
No tables used
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