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 HY000: Key 'foo' doesn't exist in table 't1' explain select * from t1 ignore key (str,str,foo) where str="foo"; ERROR HY000: Key 'foo' doesn't exist in 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 таб (кол0 int, кол1 int, key инд0 (кол0), key инд01 (кол0,кол1)); insert into таб (кол0) values (1); insert into таб (кол0) values (2); explain select кол0 from таб where кол0=1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE таб ref инд0,инд01 инд0 5 const 1 Using where; Using index drop table таб; set names latin1; select 3 into @v1; explain select 3 into @v1; 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(f1 int, f2 int); insert into t1 values (1,1); create view v1 as select * from t1 where f1=1; explain extended select * from v1 where f2=1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 system NULL NULL NULL NULL 1 Warnings: Note 1003 select '1' AS `f1`,'1' AS `f2` from `test`.`t1` where 1 explain extended select * from t1 where 0; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE Warnings: Note 1003 select `test`.`t1`.`f1` AS `f1`,`test`.`t1`.`f2` AS `f2` from `test`.`t1` where 0 explain extended select * from t1 where 1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 system NULL NULL NULL NULL 1 Warnings: Note 1003 select '1' AS `f1`,'1' AS `f2` from `test`.`t1` where 1 explain extended select * from t1 having 0; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible HAVING Warnings: Note 1003 select `test`.`t1`.`f1` AS `f1`,`test`.`t1`.`f2` AS `f2` from `test`.`t1` having 0 explain extended select * from t1 having 1; id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE t1 system NULL NULL NULL NULL 1 Warnings: Note 1003 select '1' AS `f1`,'1' AS `f2` from `test`.`t1` having 1 drop view v1; drop table t1; CREATE TABLE t1(c INT); INSERT INTO t1 VALUES (),(); CREATE TABLE t2 (b INT, KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b), KEY(b),KEY(b),KEY(b),KEY(b),KEY(b)); INSERT INTO t2 VALUES (),(),(); EXPLAIN SELECT 1 FROM (SELECT 1 FROM t2,t1 WHERE b < c GROUP BY 1 LIMIT 1) AS d2; id select_type table type possible_keys key key_len ref rows Extra X X X X X X X X X const row not found X X X X X X X X X X X X X X X X X X Range checked for each record (index map: 0xFFFFFFFFFF) DROP TABLE t2; DROP TABLE t1; CREATE TABLE t1(a INT); CREATE TABLE t2(a INT); INSERT INTO t1 VALUES (1),(2); INSERT INTO t2 VALUES (1),(2); EXPLAIN EXTENDED SELECT 1 FROM (SELECT COUNT(DISTINCT t1.a) FROM t1,t2 GROUP BY t1.a) AS s1; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY ALL NULL NULL NULL NULL 2 2 DERIVED t1 ALL NULL NULL NULL NULL 2 Using temporary; Using filesort 2 DERIVED t2 ALL NULL NULL NULL NULL 2 Warnings: Note 1003 select 1 AS `1` from (select count(distinct `test`.`t1`.`a`) AS `COUNT(DISTINCT t1.a)` from `test`.`t1` join `test`.`t2` group by `test`.`t1`.`a`) `s1` EXPLAIN EXTENDED SELECT 1 FROM (SELECT COUNT(DISTINCT t1.a) FROM t1,t2 GROUP BY t1.a) AS s1; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY ALL NULL NULL NULL NULL 2 2 DERIVED t1 ALL NULL NULL NULL NULL 2 Using temporary; Using filesort 2 DERIVED t2 ALL NULL NULL NULL NULL 2 Warnings: Note 1003 select 1 AS `1` from (select count(distinct `test`.`t1`.`a`) AS `COUNT(DISTINCT t1.a)` from `test`.`t1` join `test`.`t2` group by `test`.`t1`.`a`) `s1` prepare s1 from 'EXPLAIN EXTENDED SELECT 1 FROM (SELECT COUNT(DISTINCT t1.a) FROM t1,t2 GROUP BY t1.a) AS s1'; execute s1; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY ALL NULL NULL NULL NULL 2 2 DERIVED t1 ALL NULL NULL NULL NULL 2 Using temporary; Using filesort 2 DERIVED t2 ALL NULL NULL NULL NULL 2 Warnings: Note 1003 select 1 AS `1` from (select count(distinct `test`.`t1`.`a`) AS `COUNT(DISTINCT t1.a)` from `test`.`t1` join `test`.`t2` group by `test`.`t1`.`a`) `s1` prepare s1 from 'EXPLAIN EXTENDED SELECT 1 FROM (SELECT COUNT(DISTINCT t1.a) FROM t1,t2 GROUP BY t1.a) AS s1'; execute s1; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY ALL NULL NULL NULL NULL 2 2 DERIVED t1 ALL NULL NULL NULL NULL 2 Using temporary; Using filesort 2 DERIVED t2 ALL NULL NULL NULL NULL 2 Warnings: Note 1003 select 1 AS `1` from (select count(distinct `test`.`t1`.`a`) AS `COUNT(DISTINCT t1.a)` from `test`.`t1` join `test`.`t2` group by `test`.`t1`.`a`) `s1` execute s1; id select_type table type possible_keys key key_len ref rows Extra 1 PRIMARY ALL NULL NULL NULL NULL 2 2 DERIVED t1 ALL NULL NULL NULL NULL 2 Using temporary; Using filesort 2 DERIVED t2 ALL NULL NULL NULL NULL 2 Warnings: Note 1003 select 1 AS `1` from (select count(distinct `test`.`t1`.`a`) AS `COUNT(DISTINCT t1.a)` from `test`.`t1` join `test`.`t2` group by `test`.`t1`.`a`) `s1` DROP TABLE t1,t2; CREATE TABLE t1 (a INT PRIMARY KEY); EXPLAIN EXTENDED SELECT COUNT(a) FROM t1 USE KEY(a); ERROR HY000: Key 'a' doesn't exist in table 't1' DROP TABLE t1;