Bug #31890 Partitions: ORDER BY DESC in InnoDB not working.

It's not InnoDB specific bug.
Error is in QUEUE code, about the way we handle queue->max_at_top.
It's either '0' or '-2' and we do '^' operation to get the proper
direction. Though queue->compare() function can return '-2' as
a result of comparison sometimes. So we'll get
queue->compare() ^ queue->max_at_top == 0 (when max_at_top is -2)
and _downheap() function code will go wrong way here:
...
    if (next_index < elements &&
        (queue->compare(queue->first_cmp_arg,
                        queue->root[next_index]+offset_to_key,
                        queue->root[next_index+1]+offset_to_key) ^
         queue->max_at_top) > 0)
      next_index++;
...

Fixed by changing max_at_top to be either 1 or -1, doing
'* max_at_top' to get proper direction.

1 files changed, 27 insertions, 0 deletions

diff --git a/mysql-test/t/partition.test b/mysql-test/t/partition.test
index 524862af135..358ca5eb2cf 100644
--- a/mysql-test/t/partition.test
+++ b/mysql-test/t/partition.test
@@ -1549,4 +1549,31 @@ while ($cnt)
 --enable_query_log
 
 drop table t1;
+
+#
+# Bug #31890 Partitions: ORDER BY DESC in InnoDB not working
+#
+
+CREATE TABLE t1
+(int_column INT, char_column CHAR(5),
+PRIMARY KEY(char_column,int_column))
+PARTITION BY KEY(char_column,int_column)
+PARTITIONS 101;
+INSERT INTO t1 (int_column, char_column) VALUES
+(      39868 ,'zZZRW'),       
+(     545592 ,'zZzSD'),       
+(       4936 ,'zzzsT'),       
+(       9274 ,'ZzZSX'),       
+(     970185 ,'ZZzTN'),       
+(     786036 ,'zZzTO'),       
+(      37240 ,'zZzTv'),       
+(     313801 ,'zzzUM'),       
+(     782427 ,'ZZZva'),       
+(     907955 ,'zZZvP'),       
+(     453491 ,'zzZWV'),       
+(     756594 ,'ZZZXU'),       
+(     718061 ,'ZZzZH');       
+SELECT * FROM t1 ORDER BY char_column DESC;
+DROP TABLE t1;
+
 --echo End of 5.1 tests