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https://issues.apache.org/jira/browse/CASSANDRA-12915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15742418#comment-15742418
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Corentin Chary commented on CASSANDRA-12915:
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To give you some perspective, in my specific use case the queries are user
generated and our only safeguard is that we put a limit to the number of
generate results (5000). We already process user-provided globs into CQL
queries trying to choose the best indexes. But really it would be better if the
query optimizer was in the database rather than in our code.
I agree that my way of doing things tries to bypass a few things, but it also
doesn't seem to create regressions and provides some benefits. I propose that
we split the discussion in three parts:
1# Do you see a quick way of fixing the 'empty iterator' issue ?
2# Would you agree to chance the format to include proper cardinality
estimation to the index (so we can build upon later)
3# What would be the ETA for a proper query planner ? If O(months), would it
make sense to merge something like what I did to bring some performance
improvement in the meantime ?
Thanks !
> 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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