Question

我正在尝试使用Cassandra作为消息系统的数据库。我将所有消息保存到一个表中，该表保存了稍后我将查询的所有必需数据。现在我有一个请求，我必须显示用户最新的10个聊天室最新的100条消息。聊天室需要按照持续访问时间进行排序。

我计划创建一个表格，让我可以获得10个聊天室，并按访问时间排序。

但我无法弄清楚如何设计表格。我目前通过使用

创建表来执行此类请求

用户，聊天室，access_time。并使用（用户，聊天室）作为主键。

然后我每次收到消息时都会更新access_time。所以我可能有这样的表

user     chatroom     time
1           2          1005
2           1          1000
1           3          1003 
1           4          1004.

我很容易将用户1的列表作为

1, 2, 1005,
1, 3, 1003,
1, 4, 1004.

我自己重新订购并获得了

1, 3, 1003.
1, 4, 1004.
1, 2, 1005.

这就是我需要的最新结果。我需要[3,4,2]作为最终结果。但我无法弄清楚如何在Cassandra中进行排序部分的好方法。表有更好的设计吗？一般来说，聊天室不应该是一个大清单。因此，即使我自己排序，排序时间也不应该成为问题。

谢谢~~

埃里克

我确实尝试过Mandraenke的评论。但我的意见如下：

INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_4');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_4');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_4');
INSERT INTO room_access (user, when, room) VALUES ( 'demo4', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo5', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo7', toTimeStamp(now()), 'room_4');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo8', toTimeStamp(now()), 'room_5');
INSERT INTO room_access (user, when, room) VALUES ( 'demo9', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_7');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo5', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo7', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo7', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_5');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_8');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo5', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_4');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo7', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo4', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo6', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_1');
INSERT INTO room_access (user, when, room) VALUES ( 'demo1', toTimeStamp(now()), 'room_3');
INSERT INTO room_access (user, when, room) VALUES ( 'demo2', toTimeStamp(now()), 'room_2');
INSERT INTO room_access (user, when, room) VALUES ( 'demo3', toTimeStamp(now()), 'room_1');

我的查询结果：

select * from room_access where user='demo1';

将是

 user  | when                            | room
-------+---------------------------------+--------
 demo1 | 2018-01-01 18:17:27.886000+0000 | room_3
 demo1 | 2018-01-01 18:17:27.865000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.850000+0000 | room_2
 demo1 | 2018-01-01 18:17:27.831000+0000 | room_2
 demo1 | 2018-01-01 18:17:27.738000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.594000+0000 | room_2
 demo1 | 2018-01-01 18:17:27.556000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.513000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.456000+0000 | room_8
 demo1 | 2018-01-01 18:17:27.430000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.404000+0000 | room_3
 demo1 | 2018-01-01 18:17:27.386000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.202000+0000 | room_7
 demo1 | 2018-01-01 18:17:27.136000+0000 | room_3
 demo1 | 2018-01-01 18:17:27.088000+0000 | room_4
 demo1 | 2018-01-01 18:17:27.020000+0000 | room_2
 demo1 | 2018-01-01 18:17:26.964000+0000 | room_2
 demo1 | 2018-01-01 18:17:26.947000+0000 | room_1
 demo1 | 2018-01-01 18:14:47.267000+0000 | room_1

访问时间经常修改，实际结果将超过数千条记录（样本中只有19条记录。）但我想要的实际结果是

 user  | when                            | room
-------+---------------------------------+--------
 demo1 | 2018-01-01 18:17:27.886000+0000 | room_3
 demo1 | 2018-01-01 18:17:27.865000+0000 | room_1
 demo1 | 2018-01-01 18:17:27.850000+0000 | room_2
 demo1 | 2018-01-01 18:17:27.456000+0000 | room_8
 demo1 | 2018-01-01 18:17:27.202000+0000 | room_7
 demo1 | 2018-01-01 18:17:27.088000+0000 | room_4

似乎我仍然需要额外的过滤来获得我想要的东西。那么......有一个简单的方法来做我想要的过滤吗？

Answer 1

在Cassandra中，您几乎总是对数据进行非规范化以适合您的查询。对于您的问题，只需跟踪访问时间，例如：

cqlsh:demo> create table room_access ( user text, when timestamp, room text, primary key (user, when) ) WITH CLUSTERING ORDER BY (when DESC);

该表的唯一目的是跟踪每个用户的上次访问时间。请注意聚类列和降序：

让我们插入一些数据：

cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_a');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_b');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_c');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_d');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_e');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_f');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_g');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_h');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_i');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_j');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_k');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_l');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_b');
cqlsh:demo> INSERT INTO room_access (user, when, room) VALUES ( 'demo', toTimeStamp(now()), 'room_a');

现在您可以选择最后十个房间：

cqlsh:demo> SELECT room FROM room_access WHERE user='demo' limit 10;

 room
--------
 room_a
 room_b
 room_l
 room_k
 room_j
 room_i
 room_h
 room_g
 room_f
 room_e

(10 rows)
cqlsh:demo>

如何设计有关此类请求的表格？

1 个答案: