Bug#50843: Filesort used instead of clustered index led to

performance degradation.

Filesort + join cache combination is preferred to full index scan because it
is usually faster. But it's not the case when the index is clustered one.

Now test_if_skip_sort_order function prefers filesort only if index isn't
clustered.

mysql-test/r/innodb_mysql.result:
  Added a test case for the bug#50843.
mysql-test/t/innodb_mysql.test:
  Added a test case for the bug#50843.
sql/sql_select.cc:
  Bug#50843: Filesort used instead of clustered index led to
  performance degradation.
  Now test_if_skip_sort_order function prefers filesort only if index isn't
  clustered.

1 files changed, 13 insertions, 6 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 52f9aae7a0c..c4fac5b50ef 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -13304,12 +13304,6 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
     */
     if (select_limit >= table_records)
     {
-      /* 
-        filesort() and join cache are usually faster than reading in 
-        index order and not using join cache
-        */
-      if (tab->type == JT_ALL && tab->join->tables > tab->join->const_tables + 1)
-        DBUG_RETURN(0);
       keys= *table->file->keys_to_use_for_scanning();
       keys.merge(table->covering_keys);
 
@@ -13459,6 +13453,19 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
 	}      
       }
     }
+
+    /*
+      filesort() and join cache are usually faster than reading in 
+      index order and not using join cache, except in case that chosen
+      index is clustered primary key.
+    */
+    if ((select_limit >= table_records) &&
+        (tab->type == JT_ALL &&
+         tab->join->tables > tab->join->const_tables + 1) &&
+         ((unsigned) best_key != table->s->primary_key ||
+          !table->file->primary_key_is_clustered()))
+      DBUG_RETURN(0);
+
     if (best_key >= 0)
     {
       bool quick_created= FALSE;