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MDEV-8553: Impossible where for a!=a, a<a, a>a

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For a table column `a`, the above expressions logically
equate to false in all cases.

With this patch the optimizer knows about this and queries
like:

SELECT * FROM t1 WHERE a!=a

no longer need to evaluate a!=a for every row.

The same applies if the expression was `a<a`, or `a>a`

An `EXPLAIN SELECT COOUNT(*) FROM t1 WHERE a<a` will show:

id     select_type     table   type    possible_keys   key     key_len ref     rows    Extra
1      SIMPLE  NULL    NULL    NULL    NULL    NULL    NULL    NULL    Impossible WHERE

Similarly `NOT (a!=a)` is always true.

EXPLAIN SELECT COUNT(*) FROM t1 WHERE not (a!=a);
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
This commit is contained in:
Daniel Black
2018-12-16 17:57:47 +11:00
committed by Sergey Vojtovich
parent ad36d38024
commit d2fa5f8cfc
3 changed files with 126 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

82
mysql-test/main/type_int.result
View File

@ -280,5 +280,87 @@ id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE noticed after reading const tables
DROP TABLE t1;
#
# MDEV-8554 Expect "Impossible WHERE" for never true values like a!=a, a<a, a>a
#
CREATE TABLE t1 (a INT);
INSERT INTO t1 VALUES (1),(2),(3);
EXPLAIN SELECT * FROM t1 WHERE a!=a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
SELECT COUNT(*) FROM t1 WHERE a!=a;
COUNT(*)
0
EXPLAIN SELECT * FROM t1 WHERE a>a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
SELECT COUNT(*) FROM t1 WHERE a>a;
COUNT(*)
0
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
SELECT COUNT(*) FROM t1 WHERE a<a;
COUNT(*)
0
ALTER TABLE t1 MODIFY a TINYINT NOT NULL;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a!=a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
SELECT COUNT(*) FROM t1 WHERE a!=a;
COUNT(*)
0
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a>a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
SELECT COUNT(*) FROM t1 WHERE a>a;
COUNT(*)
0
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE NULL NULL NULL NULL NULL NULL NULL Impossible WHERE
SELECT COUNT(*) FROM t1 WHERE a<a;
COUNT(*)
0
#
# MDEV-8554 Modifing expression doesn't hit "Impossible WHERE" clause
#
EXPLAIN SELECT COUNT(*) FROM t1 WHERE not (a!=a);
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
SELECT COUNT(*) FROM t1 WHERE not (a!=a);
COUNT(*)
3
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a>a-1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
SELECT COUNT(*) FROM t1 WHERE a>a-1;
COUNT(*)
3
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a+1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
SELECT COUNT(*) FROM t1 WHERE a<a+1;
COUNT(*)
3
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a-1<a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
SELECT COUNT(*) FROM t1 WHERE a-1<a;
COUNT(*)
3
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a NOT LIKE a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
SELECT COUNT(*) FROM t1 WHERE a NOT LIKE a;
COUNT(*)
0
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a XOR a;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using where
SELECT COUNT(*) FROM t1 WHERE a XOR a;
COUNT(*)
0
DROP TABLE t1;
#
# End of 10.4 tests
#

42
mysql-test/main/type_int.test
View File

@ -206,6 +206,48 @@ EXPLAIN SELECT * FROM t1 WHERE a=200;
EXPLAIN SELECT * FROM t1 WHERE a<=>200;
DROP TABLE t1;
--echo #
--echo # MDEV-8554 Expect "Impossible WHERE" for never true values like a!=a, a<a, a>a
--echo #
CREATE TABLE t1 (a INT);
INSERT INTO t1 VALUES (1),(2),(3);
EXPLAIN SELECT * FROM t1 WHERE a!=a;
SELECT COUNT(*) FROM t1 WHERE a!=a;
EXPLAIN SELECT * FROM t1 WHERE a>a;
SELECT COUNT(*) FROM t1 WHERE a>a;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a;
SELECT COUNT(*) FROM t1 WHERE a<a;
ALTER TABLE t1 MODIFY a TINYINT NOT NULL;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a!=a;
SELECT COUNT(*) FROM t1 WHERE a!=a;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a>a;
SELECT COUNT(*) FROM t1 WHERE a>a;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a;
SELECT COUNT(*) FROM t1 WHERE a<a;
--echo #
--echo # MDEV-8554 Modifing expression doesn't hit "Impossible WHERE" clause
--echo #
EXPLAIN SELECT COUNT(*) FROM t1 WHERE not (a!=a);
SELECT COUNT(*) FROM t1 WHERE not (a!=a);
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a>a-1;
SELECT COUNT(*) FROM t1 WHERE a>a-1;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a<a+1;
SELECT COUNT(*) FROM t1 WHERE a<a+1;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a-1<a;
SELECT COUNT(*) FROM t1 WHERE a-1<a;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a NOT LIKE a;
SELECT COUNT(*) FROM t1 WHERE a NOT LIKE a;
EXPLAIN SELECT COUNT(*) FROM t1 WHERE a XOR a;
SELECT COUNT(*) FROM t1 WHERE a XOR a;
DROP TABLE t1;
--echo #
--echo # End of 10.4 tests

3
sql/sql_select.cc
View File

@ -17109,7 +17109,8 @@ Item_bool_func2::remove_eq_conds(THD *thd, Item::cond_result *cond_value,
{
if (args[0]->eq(args[1], true))
{
if (!args[0]->maybe_null || functype() == Item_func::EQUAL_FUNC)
if (*cond_value == Item::COND_FALSE ||
!args[0]->maybe_null || functype() == Item_func::EQUAL_FUNC)
return (COND*) 0; // Compare of identical items
}
}