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Bug #21456: SELECT DISTINCT(x) produces incorrect results when using order by

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GROUP BY/DISTINCT pruning optimization must be done before ORDER BY 
optimization because ORDER BY may be removed when GROUP BY/DISTINCT
sorts as a side effect, e.g. in 
  SELECT DISTINCT <non-key-col>,<pk> FROM t1
  ORDER BY <non-key-col> DISTINCT
must be removed before ORDER BY as if done the other way around
it will remove both.
This commit is contained in:
timour/timka@lamia.home 2006-08-23 16:46:57 +03:00
parent 50ae5b7989
commit de723f2998
3 changed files with 52 additions and 30 deletions
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11
mysql-test/r/distinct.result
View file

@ -555,3 +555,14 @@ EXPLAIN SELECT DISTINCT a,b,d FROM t2 GROUP BY c,b,d;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t2 ALL NULL NULL NULL NULL 3
DROP TABLE t1,t2;
CREATE TABLE t1 (a int primary key, b int);
INSERT INTO t1 (a,b) values (1,1), (2,3), (3,2);
explain SELECT DISTINCT a, b FROM t1 ORDER BY b;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 3 Using filesort
SELECT DISTINCT a, b FROM t1 ORDER BY b;
a b
1 1
3 2
2 3
DROP TABLE t1;

11
mysql-test/t/distinct.test
View file

@ -378,4 +378,15 @@ EXPLAIN SELECT DISTINCT a,b,d FROM t2 GROUP BY c,b,d;
DROP TABLE t1,t2;
#
# Bug 21456: SELECT DISTINCT(x) produces incorrect results when using order by
#
CREATE TABLE t1 (a int primary key, b int);
INSERT INTO t1 (a,b) values (1,1), (2,3), (3,2);
explain SELECT DISTINCT a, b FROM t1 ORDER BY b;
SELECT DISTINCT a, b FROM t1 ORDER BY b;
DROP TABLE t1;
# End of 4.1 tests

60
sql/sql_select.cc
View file

@ -648,6 +648,36 @@ JOIN::optimize()
if (!order && org_order)
skip_sort_order= 1;
}
/*
Check if we can optimize away GROUP BY/DISTINCT.
We can do that if there are no aggregate functions and the
fields in DISTINCT clause (if present) and/or columns in GROUP BY
(if present) contain direct references to all key parts of
an unique index (in whatever order).
Note that the unique keys for DISTINCT and GROUP BY should not
be the same (as long as they are unique).
The FROM clause must contain a single non-constant table.
*/
if (tables - const_tables == 1 && (group_list || select_distinct) &&
!tmp_table_param.sum_func_count)
{
if (group_list &&
list_contains_unique_index(join_tab[const_tables].table,
find_field_in_order_list,
(void *) group_list))
{
group_list= 0;
group= 0;
}
if (select_distinct &&
list_contains_unique_index(join_tab[const_tables].table,
find_field_in_item_list,
(void *) &fields_list))
{
select_distinct= 0;
}
}
if (group_list || tmp_table_param.sum_func_count)
{
if (! hidden_group_fields && rollup.state == ROLLUP::STATE_NONE)
@ -717,36 +747,6 @@ JOIN::optimize()
if (old_group_list && !group_list)
select_distinct= 0;
}
/*
Check if we can optimize away GROUP BY/DISTINCT.
We can do that if there are no aggregate functions and the
fields in DISTINCT clause (if present) and/or columns in GROUP BY
(if present) contain direct references to all key parts of
an unique index (in whatever order).
Note that the unique keys for DISTINCT and GROUP BY should not
be the same (as long as they are unique).
The FROM clause must contain a single non-constant table.
*/
if (tables - const_tables == 1 && (group_list || select_distinct) &&
!tmp_table_param.sum_func_count)
{
if (group_list &&
list_contains_unique_index(join_tab[const_tables].table,
find_field_in_order_list,
(void *) group_list))
{
group_list= 0;
group= 0;
}
if (select_distinct &&
list_contains_unique_index(join_tab[const_tables].table,
find_field_in_item_list,
(void *) &fields_list))
{
select_distinct= 0;
}
}
if (!group_list && group)
{
order=0; // The output has only one row