Fixed bug#35844.

The function test_if_skip_sort_order ignored any covering index used for ref
access of a table in a query with ORDER BY if this index was incompatible 
with the ORDER BY list and there was another covering index compatible with
this list. 
As a result sub-optimal execution plans were chosen for some queries with
ORDER BY clause. 

1 files changed, 6 insertions, 1 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 164edacc932..c01aaa18751 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -12966,6 +12966,7 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
     uint tablenr= tab - join->join_tab;
     ha_rows table_records= table->file->stats.records;
     bool group= join->group && order == join->group_list;
+    ha_rows ref_key_quick_rows= HA_POS_ERROR;
     LINT_INIT(best_key_parts);
     LINT_INIT(best_key_direction);
     LINT_INIT(best_records); 
@@ -12999,6 +13000,9 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
     else
       keys= usable_keys;
 
+    if (ref_key >= 0 && table->covering_keys.is_set(ref_key))
+      ref_key_quick_rows= table->quick_rows[ref_key];
+
     read_time= join->best_positions[tablenr].read_time;
     for (uint i= tablenr+1; i < join->tables; i++)
       fanout*= join->best_positions[i].records_read; // fanout is always >= 1
@@ -13093,7 +13097,8 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
               index_scan_time < read_time)
           {
             ha_rows quick_records= table_records;
-            if (is_best_covering && !is_covering)
+            if (is_best_covering && !is_covering ||
+                is_covering && ref_key_quick_rows < select_limit)
               continue;
             if (table->quick_keys.is_set(nr))
               quick_records= table->quick_rows[nr];