MDEV-9959: A serious MariaDB server performance bug10.3-MDEV-9959

Step #2: If any field in the select list of the derived tables is present in the group by list also , then we are again guaranteed that ref access to the derived table would always produce one row per key.

1 files changed, 10 insertions, 3 deletions

diff --git a/sql/table.cc b/sql/table.cc
index d2938a29ea6..f6d4a4ddc63 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -7126,10 +7126,17 @@ bool TABLE::add_tmp_key(uint key, uint key_parts,
       This handles the case when we have a single select in the derived table
     */
     st_select_lex* first= derived->first_select();
-    if ((first && !first->is_part_of_union() && 
-        first->options & SELECT_DISTINCT) ||
-        derived->check_distinct_in_union())
+    if (first)
+    {
+      if (!first->is_part_of_union() && first->options & SELECT_DISTINCT)
         keyinfo->rec_per_key[key_parts-1]=1;
+      else
+      {
+        if (derived->check_distinct_in_union() || 
+           (first->select_items_in_group_by()))
+          keyinfo->rec_per_key[key_parts-1]=1;
+      }
+    }
   }
 
   keyinfo->read_stats= NULL;