Fix datetime comparison

src/Functions/FunctionsComparison.h

@@ -1213,7 +1213,7 @@ public:
         const bool left_is_string = isStringOrFixedString(which_left);
         const bool right_is_string = isStringOrFixedString(which_right);
 
-        bool date_and_datetime = (left_type != right_type) &&
+        bool date_and_datetime = (which_left.idx != which_right.idx) &&
             which_left.isDateOrDateTime() && which_right.isDateOrDateTime();
 
         if (left_is_num && right_is_num && !date_and_datetime)

tests/performance/datetime_comparison.xml

+<test>
+    <query>SELECT count() FROM numbers(1000000000) WHERE materialize(now()) > toString(toDateTime('2020-09-30 00:00:00'))</query>
+    <query>SELECT count() FROM numbers(1000000000) WHERE materialize(now()) > toUInt32(toDateTime('2020-09-30 00:00:00'))</query>
+    <query>SELECT count() FROM numbers(1000000000) WHERE materialize(now()) > toDateTime('2020-09-30 00:00:00')</query>
+</test>