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https://issues.apache.org/jira/browse/CASSANDRA-11556?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15238773#comment-15238773
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Benjamin Lerer commented on CASSANDRA-11556:
--------------------------------------------
Even if the {{LIMIT}} clause now works like for {{SQL}} a lot of users are
still confused by it (it might be because it was implemented wrongly before
2.2).
If you have a query like {{SELECT count((*)) FROM myTable PER PARTITION LIMIT
2}} it is easy to be confused about what the result should be as it depends on
when the limit will be applied (before or after the aggregation). My guess is
that some people will believe that the limit is applied before the aggregation
and that they will open some tickets to complain about it, when they will
discover it is not. It is why in the case of {{PER PARTITION LIMIT}}, I will
prefer to throw an error. Like that users do not get wrong expectations.
3 nits regarding the patch:
* It is better to not use {{assertRowsIgnoringOrder}} for testing ordering as
it can hide some ordering problem.
* Could you also add a test for {{SELECT * FROM %s WHERE a IN (2, 3) ORDER BY b
DESC PER PARTITION LIMIT ? LIMIT ?}}?
* the {{PER PARTITION LIMIT}} tests should probably be move to
{{SelectLimitTest}}
> PER PARTITION LIMIT does not work properly for multi-partition query with
> ORDER BY
> ----------------------------------------------------------------------------------
>
> Key: CASSANDRA-11556
> URL: https://issues.apache.org/jira/browse/CASSANDRA-11556
> Project: Cassandra
> Issue Type: Bug
> Reporter: Benjamin Lerer
> Assignee: Alex Petrov
> Fix For: 3.6
>
>
> Multi-partition queries with {{PER PARTITION LIMIT}} with {{ORDER BY}} do not
> respect the {{PER PARTITION LIMIT}}.
> The problem can be reproduced with the following unit test:
> {code}
> @Test
> public void testPerPartitionLimitWithMultiPartitionQueryAndOrderBy()
> throws Throwable
> {
> createTable("CREATE TABLE %s (a int, b int, c int, PRIMARY KEY (a,
> b))");
> for (int i = 0; i < 5; i++)
> {
> for (int j = 0; j < 5; j++)
> {
> execute("INSERT INTO %s (a, b, c) VALUES (?, ?, ?)", i, j, j);
> }
> }
> assertRows(execute("SELECT * FROM %s WHERE a IN (2, 3) ORDER BY b
> DESC PER PARTITION LIMIT ?", 2),
> row(2, 4, 4),
> row(3, 4, 4),
> row(2, 3, 3),
> row(3, 3, 3));
> }
> {code}
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