Bug #47123: Endless 100% CPU loop with STRAIGHT_JOIN

 
The problem was in incorrect handling of predicates involving 
NULL as a constant value by the range optimizer. 
 
For example, when creating a SEL_ARG node from a condition of 
the form "field < const" (which would normally result in the 
"NULL < field < const" SEL_ARG),  the special case when "const" 
is NULL was not taken into account, so "NULL < field < NULL" 
was produced for the "field < NULL" condition. 
 
As a result, SEL_ARG structures of this form could not be 
further optimized which in turn could lead to incorrectly 
constructed SEL_ARG trees. In particular, code assuming SEL_ARG 
structures to always form a sequence of ordered disjoint 
intervals could enter an infinite loop under some 
circumstances. 
 
Fixed by changing get_mm_leaf() so that for any sargable 
predicate except "<=>" involving NULL as a constant, "empty" 
SEL_ARG is returned, since such a predicate is always false. 

mysql-test/r/partition_pruning.result

@@ -1272,10 +1272,9 @@ INSERT INTO t1 VALUES (1, '2009-01-01'), (2, NULL);
 # test with an invalid date, which lead to item->null_value is set.
 EXPLAIN PARTITIONS SELECT * FROM t1 WHERE b < CAST('2009-04-99' AS DATETIME);
 id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t1	p20090401	ALL	NULL	NULL	NULL	NULL	2	Using where
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 Warnings:
 Warning	1292	Incorrect datetime value: '2009-04-99'
-Warning	1292	Incorrect datetime value: '2009-04-99'
 DROP TABLE t1;
 CREATE TABLE t1
 (a INT NOT NULL AUTO_INCREMENT,

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/r/subselect.result

@@ -4403,8 +4403,7 @@ FROM t1
 WHERE a = 230;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
-2	DEPENDENT SUBQUERY	st1	index	NULL	a	5	NULL	2	Using index
-2	DEPENDENT SUBQUERY	st2	index	b	b	5	NULL	2	Using where; Using index; Using join buffer
+2	DEPENDENT SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 SELECT MAX(b), (SELECT COUNT(*) FROM st1,st2 WHERE st2.b <= t1.b)
 FROM t1 
 WHERE a = 230;

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;