[jira] [Updated] (CASSANDRA-17811) Fix CQL aggregation functions for collections, tuples and UDTs

["Andres de la Peña (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-17811?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andres de la Peña updated CASSANDRA-17811:
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    Summary: Fix CQL aggregation functions for collections, tuples and UDTs  (was: CQL functions on collections, tuples and UDTs)

> Fix CQL aggregation functions for collections, tuples and UDTs
> --------------------------------------------------------------
>
>                 Key: CASSANDRA-17811
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-17811
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: CQL/Semantics
>            Reporter: Andres de la Peña
>            Assignee: Andres de la Peña
>            Priority: Normal
>             Fix For: 4.2
>
>
> It has been found during CASSANDRA-8877 that CQLS's aggregation functions {{{}max{}}}, {{min}} and {{count}} can be applied to collections, but the result is returned as a blob. For example:
> {code:java}
> CREATE TABLE t (k int PRIMARY KEY, l list<int>);
> INSERT INTO t(k, l) VALUES (0, [1, 2, 3]);
> INSERT INTO t(k, l) VALUES (1, [10, 20, 30]);
> SELECT max(l) FROM t;
>  system.max(l)
> ------------------------------------------------------------
>  0x00000003000000040000000a0000000400000014000000040000001e
> {code}
> This happens on 3.0, 3.11, 4.0, 4.1 and trunk.
> I'm not sure on whether the function shouldn't be supported for collections, or it should be supported but the result is wrong.
> In the example above, the returned blob is the serialized value of {{{}[10, 20, 30]{}}}, which is the right one according to the list comparator. I think this happens because the matched version of the function is the one for {{{}(blob) -> blob{}}}. We would need a {{(list<int>) -> list<int>}} function instead, but this function doesn't exist.
> It would be quite easy to add versions of the {{{}max{}}}, {{min}} and {{count}} functions for every type of collection ({{{}list<int>{}}}, {{{}list<text>{}}}, {{{}map<int, int>{}}}, {{{}map<int, text>{}}}, etc.). The downside of this approach is that it would increase the number of aggregation functions kept in memory from 82 to 2722, if my maths are right. This is quite an increase, mainly due to the many possible combinations of the {{map}} type. [Here|https://github.com/adelapena/cassandra/commit/e3ba3c2dc36ce58d06942078c708ffb93eb3cd84] is a quick, incomplete prototype of the approach.
> Also, I'm not sure that applying those aggregation functions to collections is very useful in practice. Thus, an alternative approach would be just forbidding them, considering them not supported. I don't think it would be a problem for backward compatibility since no one has complained about the current behaviour, and we might well consider that the original intent was not to allow aggregation on collections. At least, there aren't any tests for it, and I can't find any documentation about it either.
> Another idea that comes to mind is that we could change the meaning of those functions to aggregate the values within the collection, instead of aggregating the rows. In that case, the behaviour would be:
> {code:java}
> CREATE TABLE t (k int PRIMARY KEY, l list<int>);
> INSERT INTO t(k, l) VALUES (0, [1, 2, 3]);
> INSERT INTO t(k, l) VALUES (1, [10, 20, 30]);
> SELECT max(l) FROM t;
>  k | system.max(l)
> ---+-----------
>  1 | 30
>  0 | 3
> {code}
> Of course we could have separate function names for that type of collection aggregations, like {{{}collectionMax{}}}, {{{}maxItem{}}}, or something like that.



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