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https://issues.apache.org/jira/browse/LUCENE-5476?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Rob Audenaerde updated LUCENE-5476:
-----------------------------------
Attachment: LUCENE-5476.patch
Here is a patch.
I'm not totally satisfied with it though. There are two loose ends:
* When a segment has many docids but very few hits, they might be skipped
altogether. If this happens in many segments, the estimate gets more and more
off
* When a segment is small, there might be a few hits that are never touched as
the randomIndex in the 'bin-o-hits' is greater than the index of the hit.
Some small performance indicators:
Using an in memory index with 10M documents, retrieving 25% of them and using a
samplingRatio of 0.001 (skipping the first call, average on the next three)
Exact: 184 ms.
Sampled: 67 ms.
Not the 10-fold increase I had hoped for, but significantly faster.
Btw. for my use case the two problems I described above are not a real issue. I
do a global count anyway, and use this to determine if the facets need to be
sampled or not. When they need to be sampled, I know for sure that there are
enough hits to give a proper estimate. The price of counting and sampling is
lower than the price of retrieving exact facets.
> 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,
> 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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