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https://issues.apache.org/jira/browse/CASSANDRA-17811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17611493#comment-17611493
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Andres de la Peña commented on CASSANDRA-17811:
-----------------------------------------------
[Here|https://github.com/adelapena/cassandra/tree/17811-trunk-collections] is
an incomplete bust mostly functional prototype of how the new mechanism for
building native functions would work to provide within-collection aggregation
functions:
{code:java}
CREATE TABLE k.t (k int PRIMARY KEY, l list<int>, m map<int, int>);
INSERT INTO t(k, l, m) VALUES (0, [1, 2, 3], {1:10, 2:20, 3:30});
> SELECT map_keys(m), map_values(m) FROM t;
system.map_keys(m) | system.map_values(m)
--------------------+----------------------
{1, 2, 3} | [10, 20, 30]
> SELECT collection_count(m), collection_count(l) FROM t;
system.collection_count(m) | system.collection_count(l)
----------------------------+----------------------------
3 | 3
> SELECT collection_min(l), collection_max(l) FROM t;
system.collection_min(l) | system.collection_max(l)
--------------------------+--------------------------
1 | 3
> SELECT collection_sum(l), collection_avg(l) FROM t;
system.collection_sum(l) | system.collection_avg(l)
--------------------------+--------------------------
6 | 2
{code}
I plan on adding those new functions on a separate ticket, but I'm presenting
them here just to show how the new mechanism for dynamically building CQL
functions would work.
The new mechanism would also be used for the new functions for dynamic data
masking (CEP-20), so we can provide masking functions for every possible data
type, including collections.
CC [~blerer]
> 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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