Bug#57095: Wrongly chosen expression cache type led to a wrong result.

The coalesce function returned DATETIME type due to a DATETIME argument, but
since it's not a date/time function it can't return correct int value for
it. Nevertheless Item_datetime_cache was chosen to cache coalesce's result
and that led to a wrong result.

Now Item_datetime_cache is used only for those function that could return
correct int representation of DATETIME values.

mysql-test/r/type_datetime.result

@@ -680,5 +680,17 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t2	const	PRIMARY	PRIMARY	8	const	1	Using index
 DROP TABLE t1,t2;
 #
+# Bug#57095: Wrongly chosen expression cache type led to a wrong
+#            result.
+#
+CREATE TABLE t1 (`b` datetime );
+INSERT INTO t1 VALUES ('2010-01-01 00:00:00'), ('2010-01-01 00:00:00');
+SELECT * FROM t1 WHERE b <= coalesce(NULL, now());
+b
+2010-01-01 00:00:00
+2010-01-01 00:00:00
+DROP TABLE t1;
+#
+#
 # End of 5.5 tests
 #

mysql-test/t/type_datetime.test

@@ -484,6 +484,16 @@ explain select * from t2 where f1=STR_TO_DATE('4/1/2010', '%m/%d/%Y');
 
 DROP TABLE t1,t2;
 
+--echo #
+--echo # Bug#57095: Wrongly chosen expression cache type led to a wrong
+--echo #            result.
+--echo #
+CREATE TABLE t1 (`b` datetime );
+INSERT INTO t1 VALUES ('2010-01-01 00:00:00'), ('2010-01-01 00:00:00');
+SELECT * FROM t1 WHERE b <= coalesce(NULL, now());
+DROP TABLE t1;
+--echo #
+
 --echo #
 --echo # End of 5.5 tests
 --echo #

sql/item.cc

@@ -7356,9 +7356,11 @@ Item_cache* Item_cache::get_cache(const Item *item, const Item_result type)
   case DECIMAL_RESULT:
     return new Item_cache_decimal();
   case STRING_RESULT:
-    if (item->field_type() == MYSQL_TYPE_DATE ||
-        item->field_type() == MYSQL_TYPE_DATETIME ||
-        item->field_type() == MYSQL_TYPE_TIME)
+    /* Not all functions that return DATE/TIME are actually DATE/TIME funcs. */
+    if ((item->field_type() == MYSQL_TYPE_DATE ||
+         item->field_type() == MYSQL_TYPE_DATETIME ||
+         item->field_type() == MYSQL_TYPE_TIME) &&
+        (const_cast<Item*>(item))->result_as_longlong())
       return new Item_cache_datetime(item->field_type());
     return new Item_cache_str(item);
   case ROW_RESULT: