Fix for bug#59308: Incorrect result for SELECT DISTINCT <col>... ORDER BY <col> DESC.

      
Also fix bug#59110: Memory leak of QUICK_SELECT_I allocated memory.
Includes Jørgen Lølands review comments.
      
Root cause of these bugs are that test_if_skip_sort_order() decided to
revert the 'skip_sort_order' descision (and use filesort) after the
query plan has been updated to reflect a 'skip' of the sort order.
      
This might happen in 'check_reverse_order:' if we have a 
select->quick which could not be made descending by appending 
a QUICK_SELECT_DESC. ().
      
The original 'save_quick' was then restored after the QEP has been modified,
which caused:
      
  - An incorrect 'precomputed_group_by= TRUE' may have been set, 
    and not reverted, as part of the already modifified QEP (Bug#59308)
  - A 'select->quick' might have been created which we fail to delete (bug#59110).
      
This fix is a refactorication of test_if_skip_sort_order() where all logic
related to modification of QEP (controlled by argument 'bool no_changes'), is
moved to the end of test_if_skip_sort_order(), and done after *all* 'test_if_skip'
checks has been performed - including the 'check_reverse_order:' checks.
      
The refactorication above contains now intentional changes to the logic which 
has been moved to the end of the function.
      
Furthermore, a smaller part of the fix address the handling of the 
select->quick objects which may already exists when we call 
'test_if_skip_sort_order()' (save_quick) -and
new select->quick's created during test_if_skip_sort_order():
      
  - Before new select->quick may be created by calling ::test_quick_select(), we
    set 'select->quick= 0' to avoid that ::test_quick_select() prematurely
    delete the save_quick's. (After this call we may have both a 'save_quick' 
    and 'select->quick')
      
  - All returns from ::test_if_skip_sort_order() where we may have both a
    'save_quick' and a 'select->quick' has been changed to goto's to the
    exit points 'skiped_sort_order:' or 'need_filesort:' where we
    decide which of the QUICK_SELECT's to keep, and delete the other.

mysql-test/r/order_by.result

@@ -1638,4 +1638,29 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t1	index	NULL	a	8	NULL	10	Using index; Using temporary; Using filesort
 1	SIMPLE	t2	eq_ref	PRIMARY	PRIMARY	4	test.t1.b	1	Using where
 DROP TABLE t1, t2;
+#
+# Bug #59110: Memory leak of QUICK_SELECT_I allocated memory 
+#  and
+# Bug #59308: Incorrect result for 
+SELECT DISTINCT <col>... ORDER BY <col> DESC 
+
+# Use Valgrind to detect #59110!
+#
+CREATE TABLE t1 (a INT,KEY (a));
+INSERT INTO t1 VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
+EXPLAIN SELECT DISTINCT a,1 FROM t1 WHERE a <> 1 ORDER BY a DESC;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	a	a	5	NULL	10	Using where; Using index; Using filesort
+SELECT DISTINCT a,1 FROM t1 WHERE a <> 1 ORDER BY a DESC;
+a	1
+10	1
+9	1
+8	1
+7	1
+6	1
+5	1
+4	1
+3	1
+2	1
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/order_by.test

@@ -1492,4 +1492,21 @@ LIMIT 2;
 DROP TABLE t1, t2;
 
 
+--echo #
+--echo # Bug #59110: Memory leak of QUICK_SELECT_I allocated memory 
+--echo #  and
+--echo # Bug #59308: Incorrect result for 
+--echo               SELECT DISTINCT <col>... ORDER BY <col> DESC 
+--echo
+--echo # Use Valgrind to detect #59110!
+--echo #
+
+CREATE TABLE t1 (a INT,KEY (a));
+INSERT INTO t1 VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
+
+EXPLAIN SELECT DISTINCT a,1 FROM t1 WHERE a <> 1 ORDER BY a DESC;
+SELECT DISTINCT a,1 FROM t1 WHERE a <> 1 ORDER BY a DESC;
+
+DROP TABLE t1;
+
 --echo End of 5.1 tests