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.

1 files changed, 14 insertions, 5 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 605ef49bb07..4c086f08af3 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -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;