alexey created SOLR-5725:
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             Summary: Efficient facets without counts for enum method
                 Key: SOLR-5725
                 URL: https://issues.apache.org/jira/browse/SOLR-5725
             Project: Solr
          Issue Type: Improvement
          Components: search
            Reporter: alexey
             Fix For: 5.0


Shot version:
This improves performance for facet.method=enum when it's enough to know that 
facet count>0, for example when you it's when you dynamically populate filters 
on search form. New method checks if two bitsets intersect instead of counting 
intersection size.


Long version:
We have a dataset containing hundreds of millions of records, we facet by 
dozens of fields with many of facet-excludes and have relatively small number 
of unique values in fields, around thousands.
Before executing search, users work with "advanced search" form, our  goal is 
to populate dozens of filters with values which are applicable with other 
selected values, so basically this is a use case for facets with mincount=1, 
but without need in actual counts.

Our performance tests showed that facet.method=enum works much better than 
fc\fcs, probably due to a specific ratio of "docset"\"unique terms count". For 
example average execution of query time with method fc=1500ms, fcs=2600ms and 
with enum=280ms. Profiling indicated the majority time for enum was spent on 
intersecting docsets.

Hers's a patch that introduces an extension to facet calculation for 
method=enum. Basically it uses docSetA.intersects(docSetB) instead of docSetA. 
intersectionSize (docSetB).

As a result we were able to reduce our average query time from 280ms to 60ms.




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