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From "Corentin Chary (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-12915) SASI: Index intersection can be very inefficient
Date Wed, 07 Dec 2016 11:14:58 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-12915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15728496#comment-15728496
] 

Corentin Chary commented on CASSANDRA-12915:
--------------------------------------------

Based on the current dataset your thinking is correct, and I agree that the obivous thing
to fix is the empty iterator.

I'll review my original notes and see if I can still find slow queries and try to explain
them individually. I strongly suspect that most of them where caused by that and that the
rest is not-significant. I'll post my findings here.

> SASI: Index intersection can be very inefficient
> ------------------------------------------------
>
>                 Key: CASSANDRA-12915
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-12915
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: sasi
>            Reporter: Corentin Chary
>             Fix For: 3.x
>
>
> It looks like RangeIntersectionIterator.java and be pretty inefficient in some cases.
Let's take the following query:
> SELECT data FROM table WHERE index1 = 'foo' AND index2 = 'bar';
> In this case:
> * index1 = 'foo' will match 2 items
> * index2 = 'bar' will match ~300k items
> On my setup, the query will take ~1 sec, most of the time being spent in disk.TokenTree.getTokenAt().
> if I patch RangeIntersectionIterator so that it doesn't try to do the intersection (and
effectively only use 'index1') the query will run in a few tenth of milliseconds.
> I see multiple solutions for that:
> * Add a static thresold to avoid the use of the index for the intersection when we know
it will be slow. Probably when the range size factor is very small and the range size is big.
> * CASSANDRA-10765



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