MDEV-27407 Different ASC/DESC index attributes on MERGE and underlying table...

MDEV-27407 Different ASC/DESC index attributes on MERGE and underlying table can cause wrong results

detect if merge children are "differently defined" regarding ASC/DESC

mysql-test/main/merge.result

@@ -3904,3 +3904,14 @@ set global default_storage_engine=@save_default_storage_engine;
 #
 # End of 10.0 tests
 #
+#
+# MDEV-27407 Different ASC/DESC index attributes on MERGE and underlying table can cause wrong results
+#
+create table t (a int, key(a desc)) engine=myisam;
+create table tm (a int, key(a)) engine=merge union(t);
+select * from tm;
+ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
+drop table tm, t;
+#
+# End of 10.8 tests
+#

mysql-test/main/merge.test

@@ -2862,3 +2862,16 @@ set global default_storage_engine=@save_default_storage_engine;
 --echo #
 --echo # End of 10.0 tests
 --echo #
+
+--echo #
+--echo # MDEV-27407 Different ASC/DESC index attributes on MERGE and underlying table can cause wrong results
+--echo #
+create table t (a int, key(a desc)) engine=myisam;
+create table tm (a int, key(a)) engine=merge union(t);
+--error ER_WRONG_MRG_TABLE
+select * from tm;
+drop table tm, t;
+
+--echo #
+--echo # End of 10.8 tests
+--echo #

storage/myisam/ha_myisam.cc

@@ -554,7 +554,8 @@ int check_definition(MI_KEYDEF *t1_keyinfo, MI_COLUMNDEF *t1_recinfo,
           t1_keysegs_j__type != t2_keysegs[j].type ||
           t1_keysegs[j].null_bit != t2_keysegs[j].null_bit ||
           t1_keysegs[j].length != t2_keysegs[j].length ||
-          t1_keysegs[j].start != t2_keysegs[j].start)
+          t1_keysegs[j].start != t2_keysegs[j].start ||
+          (t1_keysegs[j].flag ^ t2_keysegs[j].flag) & HA_REVERSE_SORT)
       {
         DBUG_PRINT("error", ("Key segment %d (key %d) has different "
                              "definition", j, i));