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https://issues.apache.org/jira/browse/CASSANDRA-17811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17634839#comment-17634839
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Andres de la Peña commented on CASSANDRA-17811:
-----------------------------------------------
Rebased and with new CI runs:
||Patch||CI||
|[trunk|https://github.com/apache/cassandra/pull/1844]|[j8|https://app.circleci.com/pipelines/github/adelapena/cassandra/2490/workflows/efd70246-1877-43a6-8ca6-11a97b0fc8f1]
[j11|https://app.circleci.com/pipelines/github/adelapena/cassandra/2490/workflows/9243ad31-5cd7-4fb1-bc12-b6329abf742a]|
|[trunk with collection
aggregation|https://github.com/apache/cassandra/compare/trunk...adelapena:cassandra:17811-trunk-collections?expand=1]|[j8|https://app.circleci.com/pipelines/github/adelapena/cassandra/2489/workflows/49bba51f-cfa8-4025-b191-a0a9c4754d61]
[j11|https://app.circleci.com/pipelines/github/adelapena/cassandra/2489/workflows/6e1b6f55-f32a-41a2-9720-2fd9e2a88d5f]|
> CQL aggregation functions on collections, tuples and UDTs
> ---------------------------------------------------------
>
> Key: CASSANDRA-17811
> URL: https://issues.apache.org/jira/browse/CASSANDRA-17811
> Project: Cassandra
> Issue Type: Bug
> Components: CQL/Semantics
> Reporter: Andres de la Peña
> Assignee: Andres de la Peña
> Priority: Normal
>
> 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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