Commit 2baf2fdf authored by unknown's avatar unknown
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Bug #21456: SELECT DISTINCT(x) produces incorrect results when using order by 

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.


mysql-test/r/distinct.result:
  Test for BUG#21456.
mysql-test/t/distinct.test:
  Test for BUG#21456.
sql/sql_select.cc:
  Bug #21456: SELECT DISTINCT(x) produces incorrect results when using order by
  
  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.
parent bfdbb780

mysql-test/r/distinct.result

+11 −0
Original line number Diff line number Diff line
@@ -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;

mysql-test/t/distinct.test

+11 −0
Original line number Diff line number Diff line
@@ -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

sql/sql_select.cc

+30 −30
Original line number Diff line number Diff line
@@ -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