BUG#15935: In mysql_update, don't use full index scan when we could have used quick select scan.

mysql-test/r/update.result:
  Testcase for BUG#15935
mysql-test/t/update.test:
  Testcase for BUG#15935
sql/sql_update.cc:
  BUG#15935: 
  - Do account for the fact that used_index!=MAX_KEY is also true for cases
    when quick select is used, and use quick select then (and not full index scan).
  - Also removed the redundant "used_index= MAX_KEY" statement

mysql-test/r/update.result

@@ -358,3 +358,18 @@ update t2,t1 set f1=3,f2=3 where f1=f2 and f1=1;
 affected rows: 3
 info: Rows matched: 3  Changed: 3  Warnings: 0
 drop table t1,t2;
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (a int, filler1 char(200), filler2 char(200), key(a));
+insert into t2 select A.a + 10*B.a, 'filler','filler' from t1 A, t1 B;
+flush status;
+update t2 set a=3 where a=2;
+show status like 'handler_read%';
+Variable_name	Value
+Handler_read_first	0
+Handler_read_key	1
+Handler_read_next	1
+Handler_read_prev	0
+Handler_read_rnd	1
+Handler_read_rnd_next	0
+drop table t1, t2;

mysql-test/t/update.test

@@ -287,4 +287,16 @@ update t1 set f1=1 where f1=3;
 update t2,t1 set f1=3,f2=3 where f1=f2 and f1=1;
 --disable_info
 drop table t1,t2;
+
+
+# BUG#15935
+create table t1 (a int);
+insert into t1 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t2 (a int, filler1 char(200), filler2 char(200), key(a));
+insert into t2 select A.a + 10*B.a, 'filler','filler' from t1 A, t1 B;
+flush status;
+update t2 set a=3 where a=2;
+show status like 'handler_read%';
+drop table t1, t2;
+
 # End of 4.1 tests

sql/sql_update.cc

@@ -212,7 +212,6 @@ int mysql_update(THD *thd,
       SORT_FIELD  *sortorder;
       ha_rows examined_rows;
 
-      used_index= MAX_KEY;                   // For call to init_read_record()
       table->sort.io_cache = (IO_CACHE *) my_malloc(sizeof(IO_CACHE),
 						    MYF(MY_FAE | MY_ZEROFILL));
       if (!(sortorder=make_unireg_sortorder(order, &length)) ||
@@ -244,7 +243,7 @@ int mysql_update(THD *thd,
 			   DISK_BUFFER_SIZE, MYF(MY_WME)))
 	goto err;
 
-      if (used_index == MAX_KEY)
+      if (used_index == MAX_KEY || (select && select->quick))
         init_read_record(&info,thd,table,select,0,1);
       else
         init_read_record_idx(&info, thd, table, 1, used_index);