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https://issues.apache.org/jira/browse/CASSANDRA-7016?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Benjamin Lerer updated CASSANDRA-7016:
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Fix Version/s: (was: 2.1.3)
3.0
Labels: cql docs (was: cql)
> can't map/reduce over subset of rows with cql
> ---------------------------------------------
>
> Key: CASSANDRA-7016
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7016
> Project: Cassandra
> Issue Type: Bug
> Components: Core, Hadoop
> Reporter: Jonathan Halliday
> Assignee: Benjamin Lerer
> Priority: Minor
> Labels: cql, docs
> Fix For: 3.0
>
> Attachments: CASSANDRA-7016-V2.txt, CASSANDRA-7016-V3.txt,
> CASSANDRA-7016.txt
>
>
> select ... where token(k) < x and token(k) >= y and k in (a,b) allow
> filtering;
> This fails on 2.0.6: can't restrict k by more than one relation.
> In the context of map/reduce (hence the token range) I want to map over only
> a subset of the keys (hence the 'in'). Pushing the 'in' filter down to cql
> is substantially cheaper than pulling all rows to the client and then
> discarding most of them.
> Currently this is possible only if the hadoop integration code is altered to
> apply the AND on the client side and use cql that contains only the resulting
> filtered 'in' set. The problem is not hadoop specific though, so IMO it
> should really be solved in cql not the hadoop integration code.
> Most restrictions on cql syntax seem to exist to prevent unduly expensive
> queries. This one seems to be doing the opposite.
> Edit: on further thought and with reference to the code in
> SelectStatement$RawStatement, it seems to me that token(k) and k should be
> considered distinct entities for the purposes of processing restrictions.
> That is, no restriction on the token should conflict with a restriction on
> the raw key. That way any monolithic query in terms of k and be decomposed
> into parallel chunks over the token range for the purposes of map/reduce
> processing simply by appending a 'and where token(k)...' clause to the
> exiting 'where k ...'.
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