Manual merge.

mysql-test/r/range.result

@@ -1398,3 +1398,12 @@ a < 10;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t3	range	a	a	5	NULL	8	Using where; Using index
 DROP TABLE t1, t2, t3;
+#
+# Bug #47123: Endless 100% CPU loop with STRAIGHT_JOIN
+#
+CREATE TABLE t1(a INT, KEY(a));
+INSERT INTO t1 VALUES (1), (NULL);
+SELECT * FROM t1 WHERE a <> NULL and (a <> NULL or a <= NULL);
+a
+DROP TABLE t1;
+End of 5.1 tests

mysql-test/t/range.test

@@ -1171,3 +1171,14 @@ a < 5 OR
 a < 10;
 
 DROP TABLE t1, t2, t3;
+
+--echo #
+--echo # Bug #47123: Endless 100% CPU loop with STRAIGHT_JOIN
+--echo #
+
+CREATE TABLE t1(a INT, KEY(a));
+INSERT INTO t1 VALUES (1), (NULL);
+SELECT * FROM t1 WHERE a <> NULL and (a <> NULL or a <= NULL);
+DROP TABLE t1;
+
+--echo End of 5.1 tests

sql/opt_range.cc

@@ -5891,6 +5891,17 @@ get_mm_leaf(RANGE_OPT_PARAM *param, COND *conf_func, Field *field,
     goto end;
   }
   field->table->in_use->variables.sql_mode= orig_sql_mode;
+
+  /*
+    Any sargable predicate except "<=>" involving NULL as a constant is always
+    FALSE
+  */
+  if (type != Item_func::EQUAL_FUNC && field->is_real_null())
+  {
+    tree= &null_element;
+    goto end;
+  }
+  
   str= (uchar*) alloc_root(alloc, key_part->store_length+1);
   if (!str)
     goto end;