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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4476) Support 2ndary index queries with only inequality clauses (LT, LTE, GT, GTE)
Date Tue, 25 Nov 2014 11:22:13 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4476?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14224410#comment-14224410
] 

Sylvain Lebresne commented on CASSANDRA-4476:
---------------------------------------------

I will note that so far for index queries we return results in partitioner order and clustering
order (within a given partition key), but this won't be the case in this issue (each index
entry has hits in partitioner order and clustering order, but we're sequentially scanning
multiple entries and so the overall result will not be in order.

I suspect we might just have to accept that, but this at least mean that:
* the patch needs to refuse {{ORDER BY}} for such queries as we can't do it without post-query
re-ordering, and post-query re-ordering doesn't work with paging so let's client do ordering
client side if they want to.
* we should carefully test the paging of those queries. I'm not sure it's broken by this but
we should have tests.

> Support 2ndary index queries with only inequality clauses (LT, LTE, GT, GTE)
> ----------------------------------------------------------------------------
>
>                 Key: CASSANDRA-4476
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4476
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: API, Core
>            Reporter: Sylvain Lebresne
>            Assignee: Oded Peer
>            Priority: Minor
>              Labels: cql
>             Fix For: 3.0
>
>         Attachments: cassandra-trunk-4476.patch
>
>
> Currently, a query that uses 2ndary indexes must have at least one EQ clause (on an indexed
column). Given that indexed CFs are local (and use LocalPartitioner that order the row by
the type of the indexed column), we should extend 2ndary indexes to allow querying indexed
columns even when no EQ clause is provided.
> As far as I can tell, the main problem to solve for this is to update KeysSearcher.highestSelectivityPredicate().
I.e. how do we estimate the selectivity of non-EQ clauses? I note however that if we can do
that estimate reasonably accurately, this might provide better performance even for index
queries that both EQ and non-EQ clauses, because some non-EQ clauses may have a much better
selectivity than EQ ones (say you index both the user country and birth date, for SELECT *
FROM users WHERE country = 'US' AND birthdate > 'Jan 2009' AND birtdate < 'July 2009',
you'd better use the birthdate index first).



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