Commit b017caef authored by unknown's avatar unknown
Browse files

Fix for BUG#21787: COUNT(*) + ORDER BY + LIMIT returns wrong result 

  
The problem was due to a prior fix for BUG 9676, which limited
the rows stored in a temporary table to the LIMIT clause. This
optimization is not applicable to non-group queries with aggregate
functions. The fix disables the optimization in this case.


mysql-test/r/limit.result:
  Test case for BUG#21787
mysql-test/t/limit.test:
  Test case for BUG#21787
sql/sql_select.cc:
  If there is an aggregate function in a non-group query,
  materialize all rows in the temporary table no matter if
  there is a LIMIT clause. This is necessary, since the
  aggregate functions must be computed over all result rows,
  not just the first LIMIT rows.
parent bfdbb780

mysql-test/r/limit.result

+14 −0
Original line number Diff line number Diff line
@@ -76,3 +76,17 @@ a 
a

1

drop table t1;

create table t1 (a int);

insert into t1 values (1),(2),(3),(4),(5),(6),(7);

explain select count(*) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	7	Using where; Using temporary

select count(*) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

c

7

explain select sum(a) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra

1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	7	Using where; Using temporary

select sum(a) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

c

28

mysql-test/t/limit.test

+10 −0
Original line number Diff line number Diff line
@@ -60,4 +60,14 @@ select 1 as a from t1 union all select 1 from dual limit 1; 
(select 1 as a from t1) union all (select 1 from dual) limit 1;

drop table t1;



#

# Bug #21787: COUNT(*) + ORDER BY + LIMIT returns wrong result

#

create table t1 (a int);

insert into t1 values (1),(2),(3),(4),(5),(6),(7);

explain select count(*) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

select count(*) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

explain select sum(a) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;

select sum(a) c FROM t1 WHERE a > 0 ORDER BY c LIMIT 3;



# End of 4.1 tests

sql/sql_select.cc

+14 −5
Original line number Diff line number Diff line
@@ -5612,11 +5612,6 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields, 
      keyinfo->key_length+=  key_part_info->length;

    }

  }

  else

  {

    set_if_smaller(table->max_rows, rows_limit);

    param->end_write_records= rows_limit;

  }



  if (distinct && field_count != param->hidden_field_count)

  {
@@ -5679,6 +5674,20 @@ create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields, 
	0 : FIELDFLAG_BINARY;

    }

  }



  /*

    Push the LIMIT clause to the temporary table creation, so that we

    materialize only up to 'rows_limit' records instead of all result records.

    This optimization is not applicable when there is GROUP BY or there is

    no GROUP BY, but there are aggregate functions, because both must be

    computed for all result rows.

  */

  if (!group && !thd->lex->current_select->with_sum_func)

  {

    set_if_smaller(table->max_rows, rows_limit);

    param->end_write_records= rows_limit;

  }



  if (thd->is_fatal_error)				// If end of memory

    goto err;					 /* purecov: inspected */

  table->db_record_offset=1;