MDEV-33318 ORDER BY COLLATE improperly applied to non-character columns

when changing charset from latin1 to utf8, adjust max_length accordingly

mysql-test/main/ctype_collate.result

@@ -772,3 +772,27 @@ string
 #
 # End of 10.2 tests
 #
+#
+# MDEV-33318 ORDER BY COLLATE improperly applied to non-character columns
+#
+set names utf8;
+create table t1 (ts datetime);
+insert t1 values ('2024-01-26 21:37:54'), ('2024-01-26 21:37:54'),
+('2024-01-26 21:37:54'), ('2024-01-26 21:37:54'),
+('2024-01-26 21:37:58'), ('2024-01-26 21:37:58'),
+('2024-01-26 21:37:58'), ('2024-01-26 21:38:02'),
+('2024-01-26 21:38:02'), ('2024-01-26 21:38:02');
+select * from t1 order by ts collate utf8_bin;
+ts
+2024-01-26 21:37:54
+2024-01-26 21:37:54
+2024-01-26 21:37:54
+2024-01-26 21:37:54
+2024-01-26 21:37:58
+2024-01-26 21:37:58
+2024-01-26 21:37:58
+2024-01-26 21:38:02
+2024-01-26 21:38:02
+2024-01-26 21:38:02
+drop table t1;
+# End of 10.6 tests

mysql-test/main/ctype_collate.test

@@ -357,3 +357,18 @@ SELECT COLUMN_GET(COLUMN_CREATE(0, 'string'),0 AS CHAR CHARACTER SET latin1 COLL
 --echo #
 --echo # End of 10.2 tests
 --echo #
+
+--echo #
+--echo # MDEV-33318 ORDER BY COLLATE improperly applied to non-character columns
+--echo #
+set names utf8;
+create table t1 (ts datetime);
+insert t1 values ('2024-01-26 21:37:54'), ('2024-01-26 21:37:54'),
+                 ('2024-01-26 21:37:54'), ('2024-01-26 21:37:54'),
+                 ('2024-01-26 21:37:58'), ('2024-01-26 21:37:58'),
+                 ('2024-01-26 21:37:58'), ('2024-01-26 21:38:02'),
+                 ('2024-01-26 21:38:02'), ('2024-01-26 21:38:02');
+select * from t1 order by ts collate utf8_bin;
+drop table t1;
+
+--echo # End of 10.6 tests

sql/item_strfunc.cc

@@ -3590,7 +3590,9 @@ bool Item_func_set_collation::fix_length_and_dec()
   }
   collation.set(m_set_collation, DERIVATION_EXPLICIT,
                 args[0]->collation.repertoire);
-  max_length= args[0]->max_length;
+  ulonglong max_char_length= (ulonglong) args[0]->max_char_length();
+  fix_char_length_ulonglong(max_char_length * collation.collation->mbmaxlen);
+
   return FALSE;
 }