Have ha_partition ignore HA_EXTRA..CHILDREN extra() calls if no myisamrg

MDEV-33502 Slowdown when running nested statement with many partitions

Optimization for tables with a lot of partitions

sql/ha_partition.cc

@@ -398,6 +398,7 @@ void ha_partition::init_handler_variables()
   m_start_key.length= 0;
   m_myisam= FALSE;
   m_innodb= FALSE;
+  m_myisammrg= FALSE;
   m_extra_cache= FALSE;
   m_extra_cache_size= 0;
   m_extra_prepare_for_update= FALSE;
@@ -3030,6 +3031,10 @@ bool ha_partition::create_handlers(MEM_ROOT *mem_root)
     DBUG_PRINT("info", ("InnoDB"));
     m_innodb= TRUE;
   }
+  else if (ha_legacy_type(hton0) == DB_TYPE_MRG_MYISAM)
+  {
+    m_myisammrg= TRUE;
+  }
   DBUG_RETURN(FALSE);
 }
 
@@ -9419,9 +9424,14 @@ int ha_partition::extra(enum ha_extra_function operation)
   }
     /* Category 9) Operations only used by MERGE */
   case HA_EXTRA_ADD_CHILDREN_LIST:
+    if (!m_myisammrg)
+      DBUG_RETURN(0);
     DBUG_RETURN(loop_partitions(extra_cb, &operation));
   case HA_EXTRA_ATTACH_CHILDREN:
   {
+    if (!m_myisammrg)
+      DBUG_RETURN(0);
+
     int result;
     uint num_locks;
     handler **file;
@@ -9440,8 +9450,9 @@ int ha_partition::extra(enum ha_extra_function operation)
     break;
   }
   case HA_EXTRA_IS_ATTACHED_CHILDREN:
-    DBUG_RETURN(loop_partitions(extra_cb, &operation));
   case HA_EXTRA_DETACH_CHILDREN:
+    if (!m_myisammrg)
+      DBUG_RETURN(0);
     DBUG_RETURN(loop_partitions(extra_cb, &operation));
   case HA_EXTRA_MARK_AS_LOG_TABLE:
   /*

sql/ha_partition.h

@@ -399,6 +399,7 @@ class ha_partition final :public handler
   */
   bool m_innodb;                        // Are all underlying handlers
                                         // InnoDB
+  bool m_myisammrg;                     // Are any of the handlers of type MERGE
   /*
     When calling extra(HA_EXTRA_CACHE) we do not pass this to the underlying
     handlers immediately. Instead we cache it and call the underlying