backport from 6.0:

Bug #40925: Equality propagation takes non indexed attribute

Query execution plans and execution time of queries like

  select a, b, c from t1
    where a > '2008-11-21' and b = a limit 10

depended on the order of equality operator parameters:
"b = a" and "a = b" are not same. 


An equality propagation algorithm has been fixed:
the substitute_for_best_equal_field function should not
substitute a field for an equal field if both fields belong
to the same table.


mysql-test/r/select.result:
  Added test case for bug #40925.
mysql-test/t/select.test:
  Added test case for bug #40925.
sql/item.cc:
  Bug #40925: Equality propagation takes non indexed attribute
  
  An equality propagation algorithm has been fixed:
  the substitute_for_best_equal_field function should not 
  substitute a field for an equal field if both fields belong
  to the same table.

mysql-test/r/select.result

@@ -4388,4 +4388,17 @@ f3	f4	count
 1	abc	1
 1	def	2
 drop table t1, t2, t3;
+CREATE TABLE t1 (a INT KEY, b INT);
+INSERT INTO t1 VALUES (1,1), (2,2), (3,3), (4,4);
+EXPLAIN EXTENDED SELECT a, b FROM t1 WHERE a > 1 AND a = b LIMIT 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	PRIMARY	PRIMARY	4	NULL	3	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` where ((`test`.`t1`.`b` = `test`.`t1`.`a`) and (`test`.`t1`.`a` > 1)) limit 2
+EXPLAIN EXTENDED SELECT a, b FROM t1 WHERE a > 1 AND b = a LIMIT 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	PRIMARY	PRIMARY	4	NULL	3	Using where
+Warnings:
+Note	1003	select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` where ((`test`.`t1`.`a` = `test`.`t1`.`b`) and (`test`.`t1`.`a` > 1)) limit 2
+DROP TABLE t1;
 End of 5.0 tests

mysql-test/t/select.test

@@ -3737,4 +3737,18 @@ cr.f4 = cr2.f4
 GROUP BY a.f3, cr.f4;
 
 drop table t1, t2, t3;
+
+
+#
+# Bug #40925: Equality propagation takes non indexed attribute
+#
+
+CREATE TABLE t1 (a INT KEY, b INT);
+INSERT INTO t1 VALUES (1,1), (2,2), (3,3), (4,4);
+
+EXPLAIN EXTENDED SELECT a, b FROM t1 WHERE a > 1 AND a = b LIMIT 2;
+EXPLAIN EXTENDED SELECT a, b FROM t1 WHERE a > 1 AND b = a LIMIT 2;
+
+DROP TABLE t1;
+ 
 --echo End of 5.0 tests

sql/item.cc

@@ -4341,7 +4341,7 @@ Item *Item_field::replace_equal_field(byte *arg)
       return const_item;
     }
     Item_field *subst= item_equal->get_first();
-    if (subst && !field->eq(subst->field))
+    if (subst && field->table != subst->field->table && !field->eq(subst->field))
       return subst;
   }
   return this;