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https://issues.apache.org/jira/browse/LUCENE-5476?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13920642#comment-13920642
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Rob Audenaerde commented on LUCENE-5476:
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Hi all, good points.
Actually, in my app, I always to a count before any other search/facetting.
This means I can set a sampling ratio to be appropriate for the number of
results (or even decide not to do sampling at all). So for my application, I
have enough functionality as-is in this issue.
But of course if would be nice if it could be implemented in one pass, because
there are more people using Lucene :)
I think it would not be that hard to implement a lower-bound; if for example
15.000 would be the minimum number of documents before sampling kicks in, the
{{RandomSamplingFacetCollector}} could also collect docs in a int[15000] to
make sure that if there are less hits, the int[] is used, else the sampled set.
The exact-part collector can stop collecting after these 15k docs.
A bit harder is the upper-bound, as you only know how many hits there are after
collecting them all. To prevent collecting all the documents, you need a
sampling rate to start skipping documents. If the result then contains too much
document; the sampled result can be re-sampled to the desired size. I think
this is more like the previous implementation of the SamplingCollector.
I have already implemented a small test (and fixed an off-by-one using it) ; I
will create a patch today that contains this test and the {{xorshift}} merged
back into the collector.
Is it a good idea to explore this upper-bound/lower-bound approach? Maybe you
already thought of alternative approaches as well?
> Facet sampling
> --------------
>
> Key: LUCENE-5476
> URL: https://issues.apache.org/jira/browse/LUCENE-5476
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Rob Audenaerde
> Attachments: LUCENE-5476.patch, LUCENE-5476.patch, LUCENE-5476.patch,
> LUCENE-5476.patch, SamplingComparison_SamplingFacetsCollector.java,
> SamplingFacetsCollector.java
>
>
> With LUCENE-5339 facet sampling disappeared.
> When trying to display facet counts on large datasets (>10M documents)
> counting facets is rather expensive, as all the hits are collected and
> processed.
> Sampling greatly reduced this and thus provided a nice speedup. Could it be
> brought back?
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