Fix for bug #34928: Confusion by having Primary Key and Index

The bug is a regression introduced in 5.1 by the patch for bug28404.

Under some circumstances test_if_skip_sort_order() could leave some
data structures in an inconsistent state so that some parts of code
could assume the selected execution strategy for GROUP BY/DISTINCT as
a loose index scan (e.g. JOIN_TAB::is_using_loose_index_scan()), while
the actual strategy chosen was an ordered index scan, which led to
wrong data being returned.

Fixed test_if_skip_sort_order() so that when changing the type for a
join table, select->quick is reset not only for EXPLAIN, but for the 
actual join execution as well, to not confuse code that depends on its
value to determine the chosen GROUP BY/DISTINCT strategy.


mysql-test/r/distinct.result:
  Added a test case for bug #34928.
mysql-test/t/distinct.test:
  Added a test case for bug #34928.
sql/sql_select.cc:
  When changing the table's join type to JT_NEXT in 
  test_if_skip_sort_order(), also reset select->quick because other
  code may depend on its value to determine the GROUP BY/DISTINCT
  execution strategy.

1 files changed, 5 insertions, 5 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 57bcb0b5942..1117df942dd 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -13158,6 +13158,11 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
           tab->read_first_record= best_key_direction > 0 ?
                                   join_read_first:join_read_last;
           tab->type=JT_NEXT;           // Read with index_first(), index_next()
+          if (select && select->quick)
+          {
+            delete select->quick;
+            select->quick= 0;
+          }
           if (table->covering_keys.is_set(best_key))
           {
             table->key_read=1;
@@ -13168,11 +13173,6 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
           {
             tab->ref.key= -1;
             tab->ref.key_parts= 0;
-            if (select && select->quick)
-            {
-              delete select->quick;
-              select->quick= 0;
-            }
             if (select_limit < table_records) 
               tab->limit= select_limit;
           }