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Bug#119442 ORDER BY optimization doesn't recognize IS NULL as constant
PROBLEM: 1. The optimizer's check_field_is_const() function only recognized equality operators (col = value) as constants for ORDER BY simplification. 2. It failed to recognize that "col IS NULL" also guarantees a single value (NULL) for sorting purposes. 3. This caused unnecessary filesort operations even when covering indexes existed. 4. Example: WHERE a = 1 AND b IS NULL ORDER BY a, b, c would use filesort despite having index (a, b, c). FIX: 1. Extended check_field_is_const() to handle Item_func::ISNULL_FUNC. 2. Added is_isnull_func() helper function to identify IS NULL conditions. 3. Properly handles OR conditions - "IS NULL OR value" remains non-constant, while "IS NULL OR IS NULL" is constant. 4. Stores IS NULL function as sentinel in const_item to ensure consistency across OR branches. 5. Preserves correct NULL ordering semantics (NULLs first in ASC, last in DESC).
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‎mysql-test/r/order_by_all.result‎

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@@ -392,7 +392,7 @@ a b c
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1 NULL b
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explain select * from t1 where a = 1 and b is null order by a desc, b desc;
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id select_type table partitions type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t1 NULL ref a a 9 const,const 2 100.00 Using where; Using index; Using filesort
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1 SIMPLE t1 NULL ref a a 9 const,const 2 100.00 Using where; Using index
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Warnings:
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Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`a` = 1) and (`test`.`t1`.`b` is null)) order by `test`.`t1`.`a` desc,`test`.`t1`.`b` desc
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select * from t1 where a = 1 and b is null order by a desc, b desc;
@@ -411,7 +411,7 @@ Warnings:
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Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`a` = 2) and (`test`.`t1`.`b` > 0)) order by `test`.`t1`.`a` desc,`test`.`t1`.`b` desc
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explain select * from t1 where a = 2 and b is null order by a desc,b desc;
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id select_type table partitions type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t1 NULL ref a a 9 const,const 1 100.00 Using where; Using index; Using filesort
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1 SIMPLE t1 NULL ref a a 9 const,const 1 100.00 Using where; Using index
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Warnings:
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Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`a` = 2) and (`test`.`t1`.`b` is null)) order by `test`.`t1`.`a` desc,`test`.`t1`.`b` desc
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explain select * from t1 where a = 2 and (b is null or b > 0) order by a

‎mysql-test/r/order_by_isnull.result‎

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#
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# Bug#119442: Optimizer doesn't simplify ORDER BY when field IS NULL
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#
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DROP TABLE IF EXISTS t1;
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CREATE TABLE t1 (
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id INT PRIMARY KEY AUTO_INCREMENT,
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category_id INT NOT NULL,
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nullable_date DATETIME DEFAULT NULL,
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created_by VARCHAR(50),
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status VARCHAR(20),
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priority TINYINT,
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item_name VARCHAR(100),
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description VARCHAR(200),
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KEY idx_covering (category_id, nullable_date, created_by, status, priority, item_name, description)
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) ENGINE=InnoDB;
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INSERT INTO t1 (category_id, nullable_date, created_by, status, priority, item_name, description) VALUES
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(100, NULL, 'User1', 'Active', 0, 'Item A', 'Description 1'),
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(100, NULL, 'User1', 'Active', 0, 'Item B', 'Description 2'),
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(100, NULL, 'User1', 'Active', 0, 'Item C', 'Description 3'),
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(100, NULL, 'User1', 'Pending', 0, 'Item D', 'Description 4'),
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(100, '2024-01-01 10:00:00', 'User1', 'Active', 0, 'Item E', 'Description 5'),
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(200, NULL, 'User2', 'Active', 1, 'Item F', 'Description 6'),
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(200, NULL, 'User3', 'Pending', 0, 'Item G', 'Description 7');
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ANALYZE TABLE t1;
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Table Op Msg_type Msg_text
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test.t1 analyze status OK
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#
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# Test 1: Basic IS NULL with ORDER BY - demonstrating covering index lookup
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# Covering index is used for IS NULL lookup (category_id, nullable_date)
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# but filesort still needed for item_name since intermediate fields aren't constant
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, nullable_date, item_name
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FROM t1 FORCE INDEX(idx_covering)
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WHERE category_id = 100 AND nullable_date IS NULL
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ORDER BY category_id, nullable_date, item_name;
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EXPLAIN
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-> Sort: t1.item_name (cost=0.775 rows=4)
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-> Filter: (t1.nullable_date is null) (cost=0.775 rows=4)
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-> Covering index lookup on t1 using idx_covering (category_id = 100, nullable_date = NULL) (cost=0.775 rows=4)
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#
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# Test 2: Compound index with IS NULL - demonstrating NO filesort
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# Unlike Test 1, all intermediate index fields are constant, allowing
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# the index to provide ordering for item_name without filesort
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, status, priority, item_name, description
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FROM t1 FORCE INDEX(idx_covering)
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WHERE category_id = 100
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AND nullable_date IS NULL
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AND created_by = 'User1'
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AND status = 'Active'
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AND priority = 0
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AND item_name >= 'Item B'
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ORDER BY category_id, nullable_date, created_by, status, priority, item_name ASC
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LIMIT 10;
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EXPLAIN
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-> Limit: 10 row(s) (cost=0.702 rows=2)
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-> Filter: ((t1.priority = 0) and (t1.`status` = 'Active') and (t1.created_by = 'User1') and (t1.category_id = 100) and (t1.nullable_date is null) and (t1.item_name >= 'Item B')) (cost=0.702 rows=2)
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-> Covering index range scan on t1 using idx_covering over (category_id = 100 AND nullable_date = NULL AND created_by = 'User1' AND status = 'Active' AND priority = 0 AND 'Item B' <= item_name) (cost=0.702 rows=2)
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SELECT id, category_id, status, priority, item_name
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FROM t1
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WHERE category_id = 100
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AND nullable_date IS NULL
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AND created_by = 'User1'
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AND status = 'Active'
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AND priority = 0
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AND item_name >= 'Item B'
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ORDER BY category_id, nullable_date, created_by, status, priority, item_name ASC
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LIMIT 10;
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id category_id status priority item_name
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2 100 Active 0 Item B
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3 100 Active 0 Item C
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#
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# Test 3: Mix of equality and IS NULL with all intermediate fields constant
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# All intermediate index fields are constant, so index can provide ordering
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# for item_name without filesort (demonstrates IS NULL works like equality)
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, nullable_date, created_by, status, priority, item_name
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FROM t1 FORCE INDEX(idx_covering)
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WHERE category_id = 100 AND nullable_date IS NULL AND created_by = 'User1' AND status = 'Active' AND priority = 0
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ORDER BY category_id, nullable_date, created_by, status, priority, item_name;
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EXPLAIN
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-> Filter: (t1.nullable_date is null) (cost=0.35 rows=1)
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-> Covering index lookup on t1 using idx_covering (category_id = 100, nullable_date = NULL, created_by = 'User1', status = 'Active', priority = 0) (cost=0.35 rows=1)
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#
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# Test 4: IS NOT NULL should NOT be treated as constant
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# This is a negative test - field can have many different non-NULL values
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, nullable_date, item_name
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FROM t1
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WHERE nullable_date IS NOT NULL
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ORDER BY nullable_date, item_name;
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EXPLAIN
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-> Sort: t1.nullable_date, t1.item_name (cost=0.95 rows=7)
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-> Filter: (t1.nullable_date is not null) (cost=0.95 rows=7)
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-> Covering index scan on t1 using idx_covering (cost=0.95 rows=7)
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#
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# Test 5: Test with optimizer trace to verify "equals_constant_in_where"
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# The trace should show that nullable_date is recognized as constant
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#
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SET optimizer_trace="enabled=on";
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SELECT id, category_id, nullable_date, item_name
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FROM t1 FORCE INDEX(idx_covering)
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WHERE category_id = 100 AND nullable_date IS NULL
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ORDER BY category_id, nullable_date, item_name
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LIMIT 2;
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id category_id nullable_date item_name
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1 100 NULL Item A
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2 100 NULL Item B
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SELECT IF(
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LOCATE('"equals_constant_in_where": true', TRACE) > 0,
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'PASS: nullable_date recognized as constant',
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'FAIL: nullable_date not recognized as constant'
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) AS trace_check
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FROM INFORMATION_SCHEMA.OPTIMIZER_TRACE;
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trace_check
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PASS: nullable_date recognized as constant
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SET optimizer_trace="enabled=off";
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#
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# Test 6: OR with IS NULL - field should NOT be treated as constant
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# (nullable_date can be NULL OR have a value, so it's not constant)
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, nullable_date, item_name
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FROM t1
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WHERE category_id = 100 AND (nullable_date IS NULL OR nullable_date > '2024-01-01')
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ORDER BY nullable_date, item_name;
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EXPLAIN
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-> Sort: t1.nullable_date, t1.item_name (cost=1.43 rows=5)
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-> Filter: ((t1.category_id = 100) and ((t1.nullable_date is null) or (t1.nullable_date > TIMESTAMP'2024-01-01 00:00:00'))) (cost=1.43 rows=5)
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-> Covering index range scan on t1 using idx_covering over (category_id = 100 AND nullable_date = NULL) OR (category_id = 100 AND '2024-01-01 00:00:00' < nullable_date) (cost=1.43 rows=5)
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SELECT id, category_id, nullable_date, item_name
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FROM t1
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WHERE category_id = 100 AND (nullable_date IS NULL OR nullable_date > '2024-01-01')
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ORDER BY nullable_date, item_name;
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id category_id nullable_date item_name
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1 100 NULL Item A
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2 100 NULL Item B
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3 100 NULL Item C
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4 100 NULL Item D
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5 100 2024-01-01 10:00:00 Item E
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#
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# Test 7: OR with multiple IS NULL - same field
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# This is effectively the same as a single IS NULL, so field IS constant
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, nullable_date, item_name
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FROM t1 FORCE INDEX(idx_covering)
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WHERE category_id = 100 AND (nullable_date IS NULL OR nullable_date IS NULL)
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ORDER BY category_id, nullable_date, item_name;
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EXPLAIN
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-> Sort: t1.item_name (cost=0.775 rows=4)
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-> Filter: ((t1.nullable_date is null) or (t1.nullable_date is null)) (cost=0.775 rows=4)
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-> Covering index lookup on t1 using idx_covering (category_id = 100, nullable_date = NULL) (cost=0.775 rows=4)
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#
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# Test 8: Verify IS NULL works with DESC ordering
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# When nullable_date IS NULL with other constants, index can still be used
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# even with DESC on the non-constant field
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#
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EXPLAIN FORMAT=TREE
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SELECT id, category_id, nullable_date, created_by, status, priority, item_name
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FROM t1 FORCE INDEX(idx_covering)
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WHERE category_id = 100 AND nullable_date IS NULL AND created_by = 'User1' AND status = 'Active' AND priority = 0
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ORDER BY category_id, nullable_date, created_by, status, priority, item_name DESC;
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EXPLAIN
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-> Filter: (t1.nullable_date is null) (cost=0.633 rows=3)
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-> Covering index lookup on t1 using idx_covering (category_id = 100, nullable_date = NULL, created_by = 'User1', status = 'Active', priority = 0) (reverse) (cost=0.633 rows=3)
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#
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# Test 9: Verify NULL ordering semantics with ASC
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# NULLs should appear FIRST with ORDER BY ASC
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#
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CREATE TABLE t_null_order (
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id INT PRIMARY KEY AUTO_INCREMENT,
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val INT,
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name VARCHAR(10),
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KEY idx_val (val, name)
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);
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INSERT INTO t_null_order (val, name) VALUES
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(NULL, 'null1'),
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(NULL, 'null2'),
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(5, 'five'),
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(10, 'ten'),
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(NULL, 'null3'),
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(1, 'one');
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ANALYZE TABLE t_null_order;
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Table Op Msg_type Msg_text
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test.t_null_order analyze status OK
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SELECT id, val, name FROM t_null_order ORDER BY val ASC, name ASC;
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id val name
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1 NULL null1
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2 NULL null2
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5 NULL null3
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6 1 one
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3 5 five
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4 10 ten
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#
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# Test 10: Verify NULL ordering semantics with DESC
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# NULLs should appear LAST with ORDER BY DESC
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#
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SELECT id, val, name FROM t_null_order ORDER BY val DESC, name ASC;
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id val name
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4 10 ten
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3 5 five
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6 1 one
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1 NULL null1
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2 NULL null2
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5 NULL null3
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#
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# Test 11: Verify optimization works with DESC and preserves NULL ordering
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# With val IS NULL, val is constant, so should only sort by name
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#
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EXPLAIN FORMAT=TREE
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SELECT id, val, name FROM t_null_order FORCE INDEX(idx_val)
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WHERE val IS NULL
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ORDER BY val DESC, name ASC;
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EXPLAIN
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-> Filter: (t_null_order.val is null) (cost=0.553 rows=3)
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-> Covering index lookup on t_null_order using idx_val (val = NULL) (cost=0.553 rows=3)
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SELECT id, val, name FROM t_null_order FORCE INDEX(idx_val)
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WHERE val IS NULL
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ORDER BY val DESC, name ASC;
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id val name
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1 NULL null1
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2 NULL null2
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5 NULL null3
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#
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# Test 12: Mixed ASC/DESC with IS NULL constraint
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#
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EXPLAIN FORMAT=TREE
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SELECT id, val, name FROM t_null_order FORCE INDEX(idx_val)
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WHERE val IS NULL
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ORDER BY val ASC, name DESC;
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EXPLAIN
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-> Filter: (t_null_order.val is null) (cost=0.553 rows=3)
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-> Covering index lookup on t_null_order using idx_val (val = NULL) (reverse) (cost=0.553 rows=3)
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SELECT id, val, name FROM t_null_order FORCE INDEX(idx_val)
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WHERE val IS NULL
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ORDER BY val ASC, name DESC;
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id val name
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5 NULL null3
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2 NULL null2
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1 NULL null1
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DROP TABLE t_null_order;
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DROP TABLE t1;
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#
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# End of test for Bug#119442
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#

‎mysql-test/r/order_by_none.result‎

Lines changed: 2 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -391,7 +391,7 @@ a b c
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1 NULL b
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explain select * from t1 where a = 1 and b is null order by a desc, b desc;
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id select_type table partitions type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t1 NULL ref a a 9 const,const 2 100.00 Using where; Using index; Using filesort
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1 SIMPLE t1 NULL ref a a 9 const,const 2 100.00 Using where; Using index
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Warnings:
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Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`a` = 1) and (`test`.`t1`.`b` is null)) order by `test`.`t1`.`a` desc,`test`.`t1`.`b` desc
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select * from t1 where a = 1 and b is null order by a desc, b desc;
@@ -410,7 +410,7 @@ Warnings:
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Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`a` = 2) and (`test`.`t1`.`b` > 0)) order by `test`.`t1`.`a` desc,`test`.`t1`.`b` desc
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explain select * from t1 where a = 2 and b is null order by a desc,b desc;
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id select_type table partitions type possible_keys key key_len ref rows filtered Extra
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1 SIMPLE t1 NULL ref a a 9 const,const 1 100.00 Using where; Using index; Using filesort
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1 SIMPLE t1 NULL ref a a 9 const,const 1 100.00 Using where; Using index
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Warnings:
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Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c` from `test`.`t1` where ((`test`.`t1`.`a` = 2) and (`test`.`t1`.`b` is null)) order by `test`.`t1`.`a` desc,`test`.`t1`.`b` desc
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explain select * from t1 where a = 2 and (b is null or b > 0) order by a

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