如何使用 Cassandra CQL 获取该表的最后 50 行？

Question

这是我用来创建表的查询：

CREATE TABLE test.comments (msguuid timeuuid, page text, userid text, username text, msg text, timestamp int, PRIMARY KEY (msguuid));

我想获取按时间戳降序排序的最后 50 行。

如果我尝试类似：SELECT * FROM test.comments WHERE page = 'test' AND timestamp < 1496468332，我会收到此错误：

Error from server: code=2200 [Invalid query] message="Cannot execute this query as it might involve data filtering and thus may have unpredictable performance. If you want to execute this query despite the performance unpredictability, use ALLOW FILTERING"

我不想使用允许过滤，我希望查询尽可能快。

我在这里查看了另一个 stackoverflow 问题 Cassandra cql: how to select the LAST n rows from a table 并尝试了解决方案：

CREATE TABLE test.comments (msguuid timeuuid, page text, userid text, username text, msg text, timestamp int, PRIMARY KEY (msguuid)) WITH CLUSTERING ORDER BY (msguuid DESC);

然后我得到这个错误：InvalidRequest: Error from server: code=2200 [Invalid query] message="Only clustering key columns can be defined in CLUSTERING ORDER directive"

我是 Cassandra 的新手，如果这有一个明显的答案，请原谅我。我似乎无法让它工作。

如果有人可以帮助我，我将不胜感激。

Answer 1

不使用索引创建Materialized View

创建一个物化视图，其中 page 作为分区键，msguuid 作为集群键 order by desc。

CREATE MATERIALIZED VIEW test.comments_by_page AS
    SELECT *
    FROM test.comments
    WHERE page IS NOT NULL AND msguuid IS NOT NULL
    PRIMARY KEY (page, msguuid)
    WITH CLUSTERING ORDER BY (msguuid DESC);

虽然您使用msguuid 作为当前时间戳的timeuuid，但您的数据将按时间降序排序。

要获取页面的最后 50 行，请使用以下查询：

SELECT * FROM comments_by_page WHERE page = 'test' LIMIT 50;

查看此链接以了解物化视图优于索引的性能以及何时不使用：http://www.datastax.com/dev/blog/materialized-view-performance-in-cassandra-3-x

Answer 2

在 cassandra 世界中，尝试根据需要满足的查询为您的表建模。如果查询总是通过 where 子句“page”并且 msguuid 只是为了唯一性而重新设计表，如下所示

CREATE TABLE test.comments (msguuid timeuuid, page text, userid text, username text, msg text, timestamp int, PRIMARY KEY (page, msguuid), WITH CLUSTERING ORDER BY (msguuid DESC));

现在表格自然是按 msguuid 排序的，不需要额外的物化视图开销。