Bug #26672:

 DATE/DATETIME values are out of the currently supported
 4 basic value types (INT,STRING,REAL and DECIMAL).
 So expressions (not fields) of compile type DATE/DATETIME are 
 generally considered as STRING values. This is not so
 when they are compared : then they are compared as 
 INTEGER values.
 But the rule for comparison as INTEGERS must be checked
 explicitly each time when a comparison is to be performed.
 filesort is one such place. However there the check was 
 not done and hence the expressions (not fields) of type 
 DATE/DATETIME were sorted by their string representation.
 Fixed to compare them as INTEGER values for filesort.

mysql-test/r/order_by.result

@@ -934,3 +934,25 @@ a	ratio
 19	1.3333
 9	2.6667
 drop table t1;
+CREATE TABLE t1 (a INT UNSIGNED NOT NULL, b TIME);
+INSERT INTO t1 (a) VALUES (100000), (0), (100), (1000000),(10000), (1000), (10);
+UPDATE t1 SET b = SEC_TO_TIME(a);
+SELECT a, b FROM t1 ORDER BY b DESC;
+a	b
+1000000	277:46:40
+100000	27:46:40
+10000	02:46:40
+1000	00:16:40
+100	00:01:40
+10	00:00:10
+0	00:00:00
+SELECT a, b FROM t1 ORDER BY SEC_TO_TIME(a) DESC;
+a	b
+1000000	277:46:40
+100000	27:46:40
+10000	02:46:40
+1000	00:16:40
+100	00:01:40
+10	00:00:10
+0	00:00:00
+DROP TABLE t1;

mysql-test/t/order_by.test

@@ -648,3 +648,18 @@ create table t1 (a int, b int, c int);
 insert into t1 values (1,2,3), (9,8,3), (19,4,3), (1,4,9);
 select a,(sum(b)/sum(c)) as ratio from t1 group by a order by sum(b)/sum(c) asc;
 drop table t1;
+
+#
+# Bug#26672: Incorrect SEC_TO_TIME() casting in ORDER BY
+#
+CREATE TABLE t1 (a INT UNSIGNED NOT NULL, b TIME);
+INSERT INTO t1 (a) VALUES (100000), (0), (100), (1000000),(10000), (1000), (10);
+UPDATE t1 SET b = SEC_TO_TIME(a);
+
+-- Correct ORDER
+SELECT a, b FROM t1 ORDER BY b DESC;
+
+-- must be ordered as the above
+SELECT a, b FROM t1 ORDER BY SEC_TO_TIME(a) DESC;
+
+DROP TABLE t1;

sql/filesort.cc

@@ -1298,7 +1298,10 @@ sortlength(THD *thd, SORT_FIELD *sortorder, uint s_length,
     }
     else
     {
-      switch ((sortorder->result_type=sortorder->item->result_type())) {
+      sortorder->result_type= sortorder->item->result_type();
+      if (sortorder->item->result_as_longlong())
+        sortorder->result_type= INT_RESULT;
+      switch (sortorder->result_type) {
       case STRING_RESULT:
 	sortorder->length=sortorder->item->max_length;
         set_if_smaller(sortorder->length, thd->variables.max_sort_length);