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https://issues.apache.org/jira/browse/SOLR-13807?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17177108#comment-17177108
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Michael Gibney commented on SOLR-13807:
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Quick follow-up: regarding skg on master vs.
77daac4ae2a4d1c40652eafbbdb42b582fe2d02d with no cache configured: the apparent
performance improvement is simultaneously a little misleading (depending on
perspective) _and also_ likely to manifest in practice for many common use
cases. This is a byproduct of the fact that in order to support caching, we
need to retain "query" references (for the purpose of building query-based
cache keys), not just domain DocSets. Once this was done, these query keys (and
in some cases extra deduping based on actual docsets) are used to ensure that
"sweep" count accumulation only accumulates once for each unique domain. So for
the common/default case where fgQ=q, and fgSet (as determined by
fgFilters=fgQ∩domainFilters) is the same as the base domain, sweep collection
only happens over the base domain (deduped with fgSet) and the bgSet -- 2
domains instead of 3. I ran the benchmarks forcing fgSet to be (slightly)
different than base domain, and performance is comparable to master (as one
would expect/hope).
> Caching for term facet counts
> -----------------------------
>
> Key: SOLR-13807
> URL: https://issues.apache.org/jira/browse/SOLR-13807
> Project: Solr
> Issue Type: New Feature
> Components: Facet Module
> Affects Versions: master (9.0), 8.2
> Reporter: Michael Gibney
> Priority: Minor
> Attachments: SOLR-13807-benchmarks.tgz,
> SOLR-13807__SOLR-13132_test_stub.patch
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Solr does not have a facet count cache; so for _every_ request, term facets
> are recalculated for _every_ (facet) field, by iterating over _every_ field
> value for _every_ doc in the result domain, and incrementing the associated
> count.
> As a result, subsequent requests end up redoing a lot of the same work,
> including all associated object allocation, GC, etc. This situation could
> benefit from integrated caching.
> Because of the domain-based, serial/iterative nature of term facet
> calculation, latency is proportional to the size of the result domain.
> Consequently, one common/clear manifestation of this issue is high latency
> for faceting over an unrestricted domain (e.g., {{\*:\*}}), as might be
> observed on a top-level landing page that exposes facets. This type of
> "static" case is often mitigated by external (to Solr) caching, either with a
> caching layer between Solr and a front-end application, or within a front-end
> application, or even with a caching layer between the end user and a
> front-end application.
> But in addition to the overhead of handling this caching elsewhere in the
> stack (or, for a new user, even being aware of this as a potential issue to
> mitigate), any external caching mitigation is really only appropriate for
> relatively static cases like the "landing page" example described above. A
> Solr-internal facet count cache (analogous to the {{filterCache}}) would
> provide the following additional benefits:
> # ease of use/out-of-the-box configuration to address a common performance
> concern
> # compact (specifically caching count arrays, without the extra baggage that
> accompanies a naive external caching approach)
> # NRT-friendly (could be implemented to be segment-aware)
> # modular, capable of reusing the same cached values in conjunction with
> variant requests over the same result domain (this would support common use
> cases like paging, but also potentially more interesting direct uses of
> facets).
> # could be used for distributed refinement (i.e., if facet counts over a
> given domain are cached, a refinement request could simply look up the
> ordinal value for each enumerated term and directly grab the count out of the
> count array that was cached during the first phase of facet calculation)
> # composable (e.g., in aggregate functions that calculate values based on
> facet counts across different domains, like SKG/relatedness – see SOLR-13132)
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