Revert the fix for bug #47123 until test suite failures are resolved.

mysql-test/r/range.result

@@ -1398,12 +1398,3 @@ 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

@@ -1171,14 +1171,3 @@ 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

@@ -5891,17 +5891,6 @@ 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;