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Bug#27573: MIN() on an indexed column which is always NULL sets _other_ results

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to NULL

For queries of the form SELECT MIN(key_part_k) FROM t1 
WHERE key_part_1 = const and ... and key_part_k-1 = const,
the opt_sum_query optimization tries to
use an index to substitute MIN/MAX functions with their values according
to the following rules:
1) Insert the minimum non-null values where the WHERE clause still matches, or
3) A row of nulls

However, the correct semantics requires that there is a third case 2)
such that a NULL value is substituted if there are only NULL values for 
key_part_k.

The patch modifies opt_sum_query() to handle this missing case.


mysql-test/r/func_group.result:
  Bug #27573: Correct result
mysql-test/t/func_group.test:
  Bug #27573: test case
sql/opt_sum.cc:
  Bug #27573:
  Added code that will try to read the
  first non-null value for a given complete-field prefix, second
  choice is to read the null, and lastly set the error code if no row
  is found.
This commit is contained in:
unknown
2007-05-15 15:29:12 +03:00
parent d03c7d2d28
commit aaf6acae9b
5 changed files with 229 additions and 10 deletions
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47
mysql-test/r/func_group.result
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@ -1321,4 +1321,51 @@ SELECT a,AVG(DISTINCT b) AS average FROM t1 GROUP BY a HAVING average > 50;
a average
1 32768.5000
DROP TABLE t1;
CREATE TABLE t1 ( a INT, b INT, KEY(a) );
INSERT INTO t1 VALUES (NULL, 1), (NULL, 2);
EXPLAIN SELECT MIN(a), MIN(b) FROM t1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 2
SELECT MIN(a), MIN(b) FROM t1;
MIN(a) MIN(b)
NULL 1
CREATE TABLE t2( a INT, b INT, c INT, KEY(a, b) );
INSERT INTO t2 ( a, b, c ) VALUES ( 1, NULL, 2 ), ( 1, 3, 4 ), ( 1, 4, 4 );
EXPLAIN SELECT MIN(b), MIN(c) FROM t2 WHERE a = 1;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 ref a a 5 const 2 Using where
SELECT MIN(b), MIN(c) FROM t2 WHERE a = 1;
MIN(b) MIN(c)
3 2
CREATE TABLE t3 (a INT, b INT, c int, KEY(a, b));
INSERT INTO t3 VALUES (1, NULL, 1), (2, NULL, 2), (2, NULL, 2), (3, NULL, 3);
EXPLAIN SELECT MIN(a), MIN(b) FROM t3 where a = 2;
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 MIN(a), MIN(b) FROM t3 where a = 2;
MIN(a) MIN(b)
2 NULL
CREATE TABLE t4 (a INT, b INT, c int, KEY(a, b));
INSERT INTO t4 VALUES (1, 1, 1), (2, NULL, 2), (2, NULL, 2), (3, 1, 3);
EXPLAIN SELECT MIN(a), MIN(b) FROM t4 where a = 2;
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 MIN(a), MIN(b) FROM t4 where a = 2;
MIN(a) MIN(b)
2 NULL
SELECT MIN(b), min(c) FROM t4 where a = 2;
MIN(b) min(c)
NULL 2
CREATE TABLE t5( a INT, b INT, KEY( a, b) );
INSERT INTO t5 VALUES( 1, 1 ), ( 1, 2 );
EXPLAIN SELECT MIN(a), MIN(b) FROM t5 WHERE a = 1;
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 MIN(a), MIN(b) FROM t5 WHERE a = 1;
MIN(a) MIN(b)
1 1
SELECT MIN(a), MIN(b) FROM t5 WHERE a = 1 and b > 1;
MIN(a) MIN(b)
1 2
DROP TABLE t1, t2, t3, t4, t5;
End of 5.0 tests