Bug #42760: Select doesn't return desired results when we have null

 values
 
 We should re-set the access method functions when changing the access
 method when switching to another index to avoid sorting.
 
 Fixed by doing a little re-engineering : encapsulating all the function
 assignment into a special function and calling it when flipping the 
 indexes.

mysql-test/r/order_by.result

@@ -1092,3 +1092,24 @@ FROM t3;
 2
 NULL
 DROP TABLE t1, t2, t3;
+#
+# Bug #42760: Select doesn't return desired results when we have null
+# values
+#
+CREATE TABLE t1 (
+a INT,
+c INT,
+UNIQUE KEY a_c (a,c),
+KEY (a));
+INSERT INTO t1 VALUES (1, 10), (2, NULL);
+# Must use ref-or-null on the a_c index
+EXPLAIN
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref_or_null	a_c,a	a_c	10	const,const	1	Using where
+# Must return 1 row
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
+col
+1
+DROP TABLE t1;
+End of 5.0 tests

mysql-test/t/order_by.test

@@ -756,3 +756,29 @@ SELECT
   FROM t3;
 
 DROP TABLE t1, t2, t3;
+
+
+
+--echo #
+--echo # Bug #42760: Select doesn't return desired results when we have null
+--echo # values
+--echo #
+
+CREATE TABLE t1 (
+  a INT,
+  c INT,
+  UNIQUE KEY a_c (a,c),
+  KEY (a));
+
+INSERT INTO t1 VALUES (1, 10), (2, NULL);
+
+--echo # Must use ref-or-null on the a_c index
+EXPLAIN
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
+--echo # Must return 1 row
+SELECT 1 AS col FROM t1 WHERE a=2 AND (c=10 OR c IS NULL) ORDER BY c;
+
+DROP TABLE t1;
+
+
+--echo End of 5.0 tests

sql/sql_select.cc

@@ -6202,6 +6202,56 @@ void rr_unlock_row(st_join_table *tab)
 
 
 
+/**
+  Pick the appropriate access method functions
+
+  Sets the functions for the selected table access method
+
+  @param      tab               Table reference to put access method
+*/
+
+static void
+pick_table_access_method(JOIN_TAB *tab)
+{
+  switch (tab->type) 
+  {
+  case JT_REF:
+    tab->read_first_record= join_read_always_key;
+    tab->read_record.read_record= join_read_next_same;
+    break;
+
+  case JT_REF_OR_NULL:
+    tab->read_first_record= join_read_always_key_or_null;
+    tab->read_record.read_record= join_read_next_same_or_null;
+    break;
+
+  case JT_CONST:
+    tab->read_first_record= join_read_const;
+    tab->read_record.read_record= join_no_more_records;
+    break;
+
+  case JT_EQ_REF:
+    tab->read_first_record= join_read_key;
+    tab->read_record.read_record= join_no_more_records;
+    break;
+
+  case JT_FT:
+    tab->read_first_record= join_ft_read_first;
+    tab->read_record.read_record= join_ft_read_next;
+    break;
+
+  case JT_SYSTEM:
+    tab->read_first_record= join_read_system;
+    tab->read_record.read_record= join_no_more_records;
+    break;
+
+  /* keep gcc happy */  
+  default:
+    break;  
+  }
+}
+
+
 static void
 make_join_readinfo(JOIN *join, ulonglong options)
 {
@@ -6234,45 +6284,15 @@ make_join_readinfo(JOIN *join, ulonglong options)
          (join->sort_by_table == (TABLE *) 1 && i != join->const_tables)))
       ordered_set= 1;
 
-    switch (tab->type) {
-    case JT_SYSTEM:				// Only happens with left join
     table->status=STATUS_NO_RECORD;
-      tab->read_first_record= join_read_system;
-      tab->read_record.read_record= join_no_more_records;
-      break;
-    case JT_CONST:				// Only happens with left join
-      table->status=STATUS_NO_RECORD;
-      tab->read_first_record= join_read_const;
-      tab->read_record.read_record= join_no_more_records;
-      if (table->used_keys.is_set(tab->ref.key) &&
-          !table->no_keyread)
-      {
-        table->key_read=1;
-        table->file->extra(HA_EXTRA_KEYREAD);
-      }
-      break;
+    pick_table_access_method (tab);
+
+    switch (tab->type) {
     case JT_EQ_REF:
-      table->status=STATUS_NO_RECORD;
-      if (tab->select)
-      {
-	delete tab->select->quick;
-	tab->select->quick=0;
-      }
-      delete tab->quick;
-      tab->quick=0;
-      tab->read_first_record= join_read_key;
       tab->read_record.unlock_row= join_read_key_unlock_row;
-      tab->read_record.read_record= join_no_more_records;
-      if (table->used_keys.is_set(tab->ref.key) &&
-	  !table->no_keyread)
-      {
-	table->key_read=1;
-	table->file->extra(HA_EXTRA_KEYREAD);
-      }
-      break;
+      /* fall through */
     case JT_REF_OR_NULL:
     case JT_REF:
-      table->status=STATUS_NO_RECORD;
       if (tab->select)
       {
 	delete tab->select->quick;
@@ -6280,34 +6300,20 @@ make_join_readinfo(JOIN *join, ulonglong options)
       }
       delete tab->quick;
       tab->quick=0;
+      /* fall through */
+    case JT_CONST:				// Only happens with left join
       if (table->used_keys.is_set(tab->ref.key) &&
 	  !table->no_keyread)
       {
 	table->key_read=1;
 	table->file->extra(HA_EXTRA_KEYREAD);
       }
-      if (tab->type == JT_REF)
-      {
-	tab->read_first_record= join_read_always_key;
-	tab->read_record.read_record= join_read_next_same;
-      }
-      else
-      {
-	tab->read_first_record= join_read_always_key_or_null;
-	tab->read_record.read_record= join_read_next_same_or_null;
-      }
-      break;
-    case JT_FT:
-      table->status=STATUS_NO_RECORD;
-      tab->read_first_record= join_ft_read_first;
-      tab->read_record.read_record= join_ft_read_next;
       break;
     case JT_ALL:
       /*
 	If previous table use cache
         If the incoming data set is already sorted don't use cache.
       */
-      table->status=STATUS_NO_RECORD;
       if (i != join->const_tables && !(options & SELECT_NO_JOIN_CACHE) &&
           tab->use_quick != 2 && !tab->first_inner && !ordered_set)
       {
@@ -6381,6 +6387,9 @@ make_join_readinfo(JOIN *join, ulonglong options)
 	}
       }
       break;
+    case JT_FT:
+    case JT_SYSTEM: 
+      break;
     default:
       DBUG_PRINT("error",("Table type %d found",tab->type)); /* purecov: deadcode */
       break;					/* purecov: deadcode */
@@ -12753,6 +12762,8 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
           if (create_ref_for_key(tab->join, tab, keyuse, 
                                  tab->join->const_table_map))
             DBUG_RETURN(0);
+
+          pick_table_access_method(tab);
 	}
 	else
 	{