Fix and test case for BUG#3649.

mysql-test/r/handler.result:
  Test case for BUG#3649
mysql-test/t/handler.test:
  Test case for BUG#3649
sql/sql_handler.cc:
  Fix for BUG#3649: when doing an index scan for an equality condition, use index_next_same to retrieve subsequent rows.

mysql-test/r/handler.result

@@ -191,3 +191,15 @@ Ok
 handler t close;
 use test;
 drop table t1;
+create table t1 ( a int, b int, INDEX a (a) );
+insert into t1 values (1,2), (2,1);
+handler t1 open;
+handler t1 read a=(1) where b=2;
+a	b
+1	2
+handler t1 read a=(1) where b=3;
+a	b
+handler t1 read a=(1) where b=1;
+a	b
+handler t1 close;
+drop table t1;

mysql-test/t/handler.test

@@ -123,3 +123,15 @@ handler t close;
 use test;
 drop table t1;
 
+#
+# BUG#3649
+#
+create table t1 ( a int, b int, INDEX a (a) );
+insert into t1 values (1,2), (2,1);
+handler t1 open;
+handler t1 read a=(1) where b=2;
+handler t1 read a=(1) where b=3;
+handler t1 read a=(1) where b=1;
+handler t1 close;
+drop table t1;
+

sql/sql_handler.cc

@@ -103,7 +103,7 @@ int mysql_ha_closeall(THD *thd, TABLE_LIST *tables)
 }
 
 static enum enum_ha_read_modes rkey_to_rnext[]=
-    { RNEXT, RNEXT, RPREV, RNEXT, RPREV, RNEXT, RPREV };
+    { RKEY, RNEXT, RPREV, RNEXT, RPREV, RNEXT, RPREV };
 
 
 int mysql_ha_read(THD *thd, TABLE_LIST *tables,
@@ -151,6 +151,10 @@ int mysql_ha_read(THD *thd, TABLE_LIST *tables,
   HANDLER_TABLES_HACK(thd);
   MYSQL_LOCK *lock=mysql_lock_tables(thd,&tables->table,1);
   HANDLER_TABLES_HACK(thd);
+  
+  byte *key= NULL;
+  uint key_len;
+  LINT_INIT(key_len); /* protected by key key variable */
   if (!lock)
      goto err0; // mysql_lock_tables() printed error message already
 
@@ -184,12 +188,17 @@ int mysql_ha_read(THD *thd, TABLE_LIST *tables,
       err=table->file->index_prev(table->record[0]);
       break;
     case RKEY:
+    {
+      if (key)
+      {
+        /* Continue scan on "(keypart1,keypart2,...)=(c1, c2, ...)  */
+        err= table->file->index_next_same(table->record[0], key, key_len);
+      }
+      else
       {
         DBUG_ASSERT(keyname != 0);
         KEY *keyinfo=table->key_info+keyno;
         KEY_PART_INFO *key_part=keyinfo->key_part;
-      uint key_len;
-      byte *key;
         if (key_expr->elements > keyinfo->key_parts)
         {
           my_printf_error(ER_TOO_MANY_KEY_PARTS,ER(ER_TOO_MANY_KEY_PARTS),
@@ -219,6 +228,7 @@ int mysql_ha_read(THD *thd, TABLE_LIST *tables,
         err=table->file->index_read(table->record[0],
                                     key,key_len,ha_rkey_mode);
         mode=rkey_to_rnext[(int)ha_rkey_mode];
+      }
       break;
     }
     default: