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

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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.
This commit is contained in:
timour/timka@lamia.home 2006-09-01 15:07:04 +03:00
parent 50ae5b7989
commit 02e194cea2
3 changed files with 38 additions and 5 deletions
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14
mysql-test/r/limit.result
View File

@ -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

10
mysql-test/t/limit.test
View File

@ -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

19
sql/sql_select.cc
View File

@ -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;