[wip] MDEV-34911

sql/item_cmpfunc.h

@@ -221,6 +221,14 @@ class Item_bool_func :public Item_int_func,
     if (field->type() == Item::FIELD_ITEM && !field->const_item() &&
         (!value || !value->is_expensive()))
       DBUG_RETURN(get_full_func_mm_tree(param, (Item_field *) field, value));
+    if ((field->type() == Item::FUNC_ITEM &&
+         !strcmp(((Item_func *) field)->func_name(), "substr") &&
+          ((Item_func *) field)->arguments()[1]->val_int() == 1) &&
+        !field->const_item() && (!value || !value->is_expensive()))
+      DBUG_RETURN(get_full_func_mm_tree(
+                    param,
+                    (Item_field *) ((Item_func *) field)->arguments()[0],
+                    value));
     DBUG_RETURN(NULL);
   }
   SEL_TREE *get_mm_parts(RANGE_OPT_PARAM *param, Field *field,

sql/sql_select.cc

@@ -7080,6 +7080,15 @@ Item_bool_func2::add_key_fields_optimize_op(JOIN *join, KEY_FIELD **key_fields,
                          (Item_field*) args[0]->real_item(), equal_func,
                          args + 1, 1, usable_tables, sargables);
   }
+  else if (args[0]->type() == FUNC_ITEM && args[1]->type() == CONST_ITEM &&
+           equal_func)
+  {
+    if (!strcmp(((Item_func *) args[0])->func_name(), "substr") &&
+        ((Item_func_substr *) args[0])->arguments()[1]->val_int() == 1)
+      add_key_equal_fields(join, key_fields, *and_level, this,
+                           ((Item_func_substr *) args[0])->arguments()[0]->real_item(),
+                           false, args + 1, 1, usable_tables, sargables);
+  }
   if (is_local_field(args[1]))
   {
     add_key_equal_fields(join, key_fields, *and_level, this,