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
 
@@ -185,40 +189,46 @@ int mysql_ha_read(THD *thd, TABLE_LIST *tables,
       break;
     case RKEY:
     {
-      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)
+      if (key)
       {
-	my_printf_error(ER_TOO_MANY_KEY_PARTS,ER(ER_TOO_MANY_KEY_PARTS),
-			MYF(0),keyinfo->key_parts);
-	goto err;
+        /* Continue scan on "(keypart1,keypart2,...)=(c1, c2, ...)  */
+        err= table->file->index_next_same(table->record[0], key, key_len);
       }
-      List_iterator_fast<Item> it_ke(*key_expr);
-      Item *item;
-      for (key_len=0 ; (item=it_ke++) ; key_part++)
+      else
       {
-	if (item->fix_fields(thd, tables))
-	  goto err;
-	if (item->used_tables() & ~RAND_TABLE_BIT)
+        DBUG_ASSERT(keyname != 0);
+        KEY *keyinfo=table->key_info+keyno;
+        KEY_PART_INFO *key_part=keyinfo->key_part;
+        if (key_expr->elements > keyinfo->key_parts)
         {
-          my_error(ER_WRONG_ARGUMENTS,MYF(0),"HANDLER ... READ");
-	  goto err;
+          my_printf_error(ER_TOO_MANY_KEY_PARTS,ER(ER_TOO_MANY_KEY_PARTS),
+                          MYF(0),keyinfo->key_parts);
+          goto err;
         }
-	item->save_in_field(key_part->field, 1);
-	key_len+=key_part->store_length;
-      }
-      if (!(key= (byte*) thd->calloc(ALIGN_SIZE(key_len))))
-      {
-	send_error(&thd->net,ER_OUTOFMEMORY);
-	goto err;
+        List_iterator_fast<Item> it_ke(*key_expr);
+        Item *item;
+        for (key_len=0 ; (item=it_ke++) ; key_part++)
+        {
+          if (item->fix_fields(thd, tables))
+            goto err;
+          if (item->used_tables() & ~RAND_TABLE_BIT)
+          {
+            my_error(ER_WRONG_ARGUMENTS,MYF(0),"HANDLER ... READ");
+            goto err;
+          }
+          item->save_in_field(key_part->field, 1);
+          key_len+=key_part->store_length;
+        }
+        if (!(key= (byte*) thd->calloc(ALIGN_SIZE(key_len))))
+        {
+          send_error(&thd->net,ER_OUTOFMEMORY);
+          goto err;
+        }
+        key_copy(key, table, keyno, key_len);
+        err=table->file->index_read(table->record[0],
+                                    key,key_len,ha_rkey_mode);
+        mode=rkey_to_rnext[(int)ha_rkey_mode];
       }
-      key_copy(key, table, keyno, key_len);
-      err=table->file->index_read(table->record[0],
-				  key,key_len,ha_rkey_mode);
-      mode=rkey_to_rnext[(int)ha_rkey_mode];
       break;
     }
     default: