Bug#46454: MySQL wrong index optimisation leads to incorrect result & crashes

Problem 1:
When the 'Using index' optimization is used, the optimizer may still - after
cost-based optimization - decide to use another index in order to avoid using
a temporary table. But when this happens, the flag to the storage engine to 
read index only (not table) was still set. Fixed by resetting the flag in the 
storage engine and TABLE structure in the above scenario, unless the new index
allows for the same optimization.
Problem 2:
When a 'ref' access method was employed by cost-based optimizer, (when the column
is non-NULLable), it was assumed that it needed no initialization if 'quick' access
methods (since they are based on range scan). When ORDER BY optimization overrides 
the decision, however, it expects to have this initialized and hence crashes. 
Fixed in 5.1 (was fixed in 6.0 already) by initializing 'quick' even when there's 
'ref' access. 

mysql-test/r/order_by.result:
  Bug#46454: Test result.
mysql-test/t/order_by.test:
  Bug#46454: Test case.
sql/sql_select.cc:
  Bug#46454: 
  Problem 1 fixed in make_join_select()
  Problem 2 fixed in test_if_skip_sort_order()
sql/table.h:
  Bug#46454: Added comment to field.

mysql-test/r/order_by.result

@@ -1500,3 +1500,60 @@ id1
 15
 16
 DROP TABLE t1;
+CREATE TABLE t1 (
+a INT,
+b INT NOT NULL,
+c char(100),
+KEY (b, c),
+KEY (b, a, c)
+)
+DEFAULT CHARSET = utf8;
+INSERT INTO t1 VALUES 
+(1,  1, 1),
+(2,  2, 2),
+(3,  3, 3),
+(4,  4, 4),
+(5,  5, 5),
+(6,  6, 6),
+(7,  7, 7),
+(8,  8, 8),
+(9,  9, 9);
+INSERT INTO t1 SELECT a + 10,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 20,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 40,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 80,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 160, b, c FROM t1;
+INSERT INTO t1 SELECT a + 320, b, c FROM t1;
+INSERT INTO t1 SELECT a + 640, b, c FROM t1;
+INSERT INTO t1 SELECT a + 1280, b, c FROM t1 LIMIT 80;
+EXPLAIN
+SELECT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 9;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	b,b_2	b	4	NULL	226	Using where
+SELECT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 9;
+a
+2071
+2061
+2051
+2041
+2031
+2021
+2011
+2001
+1991
+EXPLAIN
+SELECT DISTINCT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 0, 9;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	b,b_2	b	4	NULL	226	Using where; Using temporary
+SELECT DISTINCT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 0, 9;
+a
+2071
+2061
+2051
+2041
+2031
+2021
+2011
+2001
+1991
+DROP TABLE t1;

mysql-test/t/order_by.test

@@ -1361,3 +1361,44 @@ DROP TABLE t1;
 
 
 
+#
+# Bug#46454: MySQL wrong index optimisation leads to incorrect result & crashes 
+#
+CREATE TABLE t1 (
+  a INT,
+  b INT NOT NULL,
+  c char(100),
+  KEY (b, c),
+  KEY (b, a, c)
+)
+DEFAULT CHARSET = utf8;
+
+INSERT INTO t1 VALUES 
+(1,  1, 1),
+(2,  2, 2),
+(3,  3, 3),
+(4,  4, 4),
+(5,  5, 5),
+(6,  6, 6),
+(7,  7, 7),
+(8,  8, 8),
+(9,  9, 9);
+
+INSERT INTO t1 SELECT a + 10,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 20,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 40,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 80,  b, c FROM t1;
+INSERT INTO t1 SELECT a + 160, b, c FROM t1;
+INSERT INTO t1 SELECT a + 320, b, c FROM t1;
+INSERT INTO t1 SELECT a + 640, b, c FROM t1;
+INSERT INTO t1 SELECT a + 1280, b, c FROM t1 LIMIT 80;
+
+EXPLAIN
+SELECT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 9;
+SELECT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 9;
+
+EXPLAIN
+SELECT DISTINCT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 0, 9;
+SELECT DISTINCT a FROM t1 WHERE b = 1 ORDER BY c DESC LIMIT 0, 9;
+
+DROP TABLE t1;

sql/sql_select.cc

@@ -6151,7 +6151,7 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
         }
 
       }
-      if (tmp || !cond)
+      if (tmp || !cond || tab->type == JT_REF)
       {
         DBUG_EXECUTE("where",print_where(tmp,tab->table->alias, QT_ORDINARY););
 	SQL_SELECT *sel= tab->select= ((SQL_SELECT*)
@@ -6165,7 +6165,7 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond)
           The guard will turn the predicate on only after
           the first match for outer tables is encountered.
 	*/        
-        if (cond)
+        if (cond && tmp)
         {
           /*
             Because of QUICK_GROUP_MIN_MAX_SELECT there may be a select without
@@ -12946,6 +12946,8 @@ find_field_in_item_list (Field *field, void *data)
 
   The index must cover all fields in <order>, or it will not be considered.
 
+  @param no_changes No changes will be made to the query plan.
+
   @todo
     - sergeyp: Results of all index merge selects actually are ordered 
     by clustered PK values.
@@ -13280,6 +13282,15 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
       }
       if (!no_changes)
       {
+        /* 
+           If ref_key used index tree reading only ('Using index' in EXPLAIN),
+           and best_key doesn't, then revert the decision.
+        */
+        if (!table->covering_keys.is_set(best_key) && table->key_read)
+        {
+          table->key_read= 0;
+          table->file->extra(HA_EXTRA_NO_KEYREAD);          
+        }        
         if (!quick_created)
 	{
           tab->index= best_key;
@@ -13296,16 +13307,6 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
             table->key_read=1;
             table->file->extra(HA_EXTRA_KEYREAD);
           }
-          else if (table->key_read)
-          {
-            /*
-              Clear the covering key read flags that might have been
-              previously set for some key other than the current best_key.
-            */
-            table->key_read= 0;
-            table->file->extra(HA_EXTRA_NO_KEYREAD);
-          }
-
           table->file->ha_index_or_rnd_end();
           if (join->select_options & SELECT_DESCRIBE)
           {

sql/table.h

@@ -755,7 +755,13 @@ struct st_table {
   */
   my_bool force_index;
   my_bool distinct,const_table,no_rows;
-  my_bool key_read, no_keyread;
+
+  /**
+     If set, the optimizer has found that row retrieval should access index 
+     tree only.
+   */
+  my_bool key_read;
+  my_bool no_keyread;
   /*
     Placeholder for an open table which prevents other connections
     from taking name-locks on this table. Typically used with