drop table if exists t1; create table t1 (id int not null, str char(10), unique(str)); explain select * from t1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 system NULL NULL NULL NULL 0 const row not found 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; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 ref str str 11 const 1 Using where explain select * from t1 where str="foo"; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 const str str 11 const 1 explain select * from t1 ignore key (str) where str="foo"; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 ALL NULL NULL NULL NULL 4 Using where explain select * from t1 use key (str,str) where str="foo"; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 const str str 11 const 1 explain select * from t1 use key (str,str,foo) where str="foo"; ERROR 42000: Index 'foo' is not defined for table 't1' explain select * from t1 ignore key (str,str,foo) where str="foo"; ERROR 42000: Index 'foo' is not defined for table 't1' drop table t1; explain select 1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL No tables used create table t1 (a int not null); explain select count(*) from t1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away insert into t1 values(1); explain select count(*) from t1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away insert into t1 values(1); explain select count(*) from t1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Select tables optimized away drop table t1; set names koi8r; create table ��� (���0 int, ���1 int, key ���0 (���0), key ���01 (���0,���1)); insert into ��� (���0) values (1); insert into ��� (���0) values (2); explain select ���0 from ��� where ���0=1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE ��� ref ���0,���01 ���0 5 const 1 Using where; Using index drop table ���; set names latin1;