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https://issues.apache.org/jira/browse/SOLR-5894?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Toke Eskildsen updated SOLR-5894:
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Attachment: author_7M_tags_1852_logged_queries_warmed.png
Results from a small scale test of 2 threads sending logged queries against our
~50GB, 11M document library index, faceting on 'author' with 7M unique values.
The test was executed multiple times and the last (and thus fully warmed)
results were used. Only searches with facet results were counted.
Results for this particular test were median response times of 36ms for
standard facet counting and 24ms for sparse. See the attached PNG for graphs.
Your Mileage Will Vary.
> Speed up high-cardinality facets with sparse counters
> -----------------------------------------------------
>
> Key: SOLR-5894
> URL: https://issues.apache.org/jira/browse/SOLR-5894
> Project: Solr
> Issue Type: Improvement
> Components: SearchComponents - other
> Affects Versions: 4.6.1, 4.7
> Reporter: Toke Eskildsen
> Priority: Minor
> Fix For: 4.6.1
>
> Attachments: SOLR-5894.patch,
> author_7M_tags_1852_logged_queries_warmed.png
>
>
> Field based faceting in Solr has two phases: Collecting counts for tags in
> facets and extracting the requested tags.
> The execution time for the collecting phase is approximately linear to the
> number of hits and the number of references from hits to tags. This phase is
> not the focus here.
> The extraction time scales with the number of unique tags in the search
> result, but is also heavily influenced by the total number of unique tags in
> the facet as every counter, 0 or not, is visited by the extractor (at least
> for count order). For fields with millions of unique tag values this means
> 10s of milliseconds added to the minimum response time (see
> https://sbdevel.wordpress.com/2014/03/18/sparse-facet-counting-on-a-real-index/
> for a test on a corpus with 7M unique values in the facet).
> The extractor needs to visit every counter due to the current counter
> structure being a plain int-array of size #unique_tags. Switching to a sparse
> structure, where only the tag counters > 0 are visited, makes the extraction
> time linear to the number of unique tags in the result set.
> Unfortunately the number of unique tags in the result set is unknown at
> collect time, so it is not possible to reliably select sparse counting vs.
> full counting up front. Luckily there exists solutions for sparse sets that
> has the property of switching to non-sparse-mode without a switch-penalty,
> when the sparse-threshold is exceeded (see
> http://programmingpraxis.com/2012/03/09/sparse-sets/ for an example). This
> JIRA aims to implement this functionality in Solr (a proof of concept patch
> will be provided shortly).
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