Fix for BUG#7331.

The problem was that when a QUICK_SELECT access method is chosen,
test_if_skip_sort_order() discovered that the index being used
by the quick select will not deliver tuples in sorted order.
In this case test_if_skip_sort_order() tried to change the index
used by the quick select, but it didn't properly set the other
members of the quick select, and especially the range flags of
the ranges in QUICK_SELECT::ranges.

The fix re-invokes the function SQL_SELECT::test_quick_select
to correctly create a valid QUICK_SELECT object.

mysql-test/r/order_by.result

@@ -733,3 +733,41 @@ xxxxxxxxxxxxxxxxxxxaa
 xxxxxxxxxxxxxxxxxxxx
 xxxxxxxxxxxxxxxxxxxz
 drop table t1;
+create table t1 (
+`sid` decimal(8,0) default null,
+`wnid` varchar(11) not null default '',
+key `wnid14` (`wnid`(4)),
+key `wnid` (`wnid`)
+) engine=myisam default charset=latin1;
+insert into t1 (`sid`, `wnid`) values
+('10100','01019000000'),('37986','01019000000'),('37987','01019010000'),
+('39560','01019090000'),('37989','01019000000'),('37990','01019011000'),
+('37991','01019011000'),('37992','01019019000'),('37993','01019030000'),
+('37994','01019090000'),('475','02070000000'),('25253','02071100000'),
+('25255','02071100000'),('25256','02071110000'),('25258','02071130000'),
+('25259','02071190000'),('25260','02071200000'),('25261','02071210000'),
+('25262','02071290000'),('25263','02071300000'),('25264','02071310000'),
+('25265','02071310000'),('25266','02071320000'),('25267','02071320000'),
+('25269','02071330000'),('25270','02071340000'),('25271','02071350000'),
+('25272','02071360000'),('25273','02071370000'),('25281','02071391000'),
+('25282','02071391000'),('25283','02071399000'),('25284','02071400000'),
+('25285','02071410000'),('25286','02071410000'),('25287','02071420000'),
+('25288','02071420000'),('25291','02071430000'),('25290','02071440000'),
+('25292','02071450000'),('25293','02071460000'),('25294','02071470000'),
+('25295','02071491000'),('25296','02071491000'),('25297','02071499000');
+explain select * from t1 where wnid like '0101%' order by wnid;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	range	wnid14,wnid	wnid	11	NULL	10	Using where
+select * from t1 where wnid like '0101%' order by wnid;
+sid	wnid
+10100	01019000000
+37986	01019000000
+37989	01019000000
+37987	01019010000
+37990	01019011000
+37991	01019011000
+37992	01019019000
+37993	01019030000
+39560	01019090000
+37994	01019090000
+drop table t1;

mysql-test/t/order_by.test

@@ -500,3 +500,37 @@ insert into t1 set a = concat(repeat('x', 19), 'aa');
 set max_sort_length=20;
 select a from t1 order by a;
 drop table t1;
+
+#
+# Bug #7331
+#
+
+create table t1 (
+  `sid` decimal(8,0) default null,
+  `wnid` varchar(11) not null default '',
+  key `wnid14` (`wnid`(4)),
+  key `wnid` (`wnid`)
+) engine=myisam default charset=latin1;
+
+insert into t1 (`sid`, `wnid`) values
+('10100','01019000000'),('37986','01019000000'),('37987','01019010000'),
+('39560','01019090000'),('37989','01019000000'),('37990','01019011000'),
+('37991','01019011000'),('37992','01019019000'),('37993','01019030000'),
+('37994','01019090000'),('475','02070000000'),('25253','02071100000'),
+('25255','02071100000'),('25256','02071110000'),('25258','02071130000'),
+('25259','02071190000'),('25260','02071200000'),('25261','02071210000'),
+('25262','02071290000'),('25263','02071300000'),('25264','02071310000'),
+('25265','02071310000'),('25266','02071320000'),('25267','02071320000'),
+('25269','02071330000'),('25270','02071340000'),('25271','02071350000'),
+('25272','02071360000'),('25273','02071370000'),('25281','02071391000'),
+('25282','02071391000'),('25283','02071399000'),('25284','02071400000'),
+('25285','02071410000'),('25286','02071410000'),('25287','02071420000'),
+('25288','02071420000'),('25291','02071430000'),('25290','02071440000'),
+('25292','02071450000'),('25293','02071460000'),('25294','02071470000'),
+('25295','02071491000'),('25296','02071491000'),('25297','02071499000');
+
+explain select * from t1 where wnid like '0101%' order by wnid;
+
+select * from t1 where wnid like '0101%' order by wnid;
+
+drop table t1;

sql/sql_select.cc

@@ -7153,9 +7153,22 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
 	}
 	else
 	{
-          select->quick->file->ha_index_end();
-	  select->quick->index= new_ref_key;
-	  select->quick->init();
+          /*
+            The range optimizer constructed QUICK_RANGE for ref_key, and
+            we want to use instead new_ref_key as the index. We can't
+            just change the index of the quick select, because this may
+            result in an incosistent QUICK_SELECT object. Below we
+            create a new QUICK_SELECT from scratch so that all its
+            parameres are set correctly by the range optimizer.
+           */
+          key_map new_ref_key_map;
+          new_ref_key_map.clear_all();  /* Force the creation of quick select */
+          new_ref_key_map.set_bit(new_ref_key); /* only for new_ref_key.      */
+
+          if (select->test_quick_select(tab->join->thd, new_ref_key_map, 0,
+                                        (tab->join->select_options & OPTION_FOUND_ROWS) ?
+                                        HA_POS_ERROR : tab->join->unit->select_limit_cnt) <= 0)
+            DBUG_RETURN(0);
 	}
         ref_key= new_ref_key;
       }