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https://issues.apache.org/jira/browse/SOLR-7461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14528822#comment-14528822
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Hoss Man commented on SOLR-7461:
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James: sorry, i overlooked your comments until now...
bq. Regarding the hyperloglog approach:
please raise HLL related discussions in the comments of SOLR-6968 - been making
lots of progress there, would love your opinions & help testing, but i don't
want to have the conversation fractured in two diff places for other people
watching that issue.
----
bq. Unfortunately, for me, being able to get exact (or very nearly exact)
distinct counts is important for the application I'm building (it's an
analytics / stats tool so correct numbers are a must have feature)
I understand ... the reason i popped back up in this issue today was to comment
that:
1) as I work more with HLL and really think about the internals, i realized
that for people who care a lot about absolutely accurate precision, especially
in low cardinality situations, i think you are correct -- fixing this issue
(countdistinct w/o distinctValues) would be a big win -- even once HLL support
is added. The simple reason being that HLL is fundamentally based on counting
unique *hashed* values, so even if someone tries to tune it to be as accurate
as possible, there's still the risk that two distinct values in a users's set
will have the same hash value and prevent 100% accurate cardinality results.
2) as i worked on refactoring the tests in SOLR-6968 it occurred to me that
what you're requesting here would actually be fairly easy to implement and
support in a backcompat way -- in particular because of the changes already
made in SOLR-6349 (distinguishing per-shard dependent stats) and the fact that
the current param is "calcdistinct" but the stats currently returned are
"countDistinct" and "distinctValues"
In a nutshell, what i think we should do is...
* replace {{calcdistinct(true)}} value in the {{Stat}} enum with
{{distinctvals(true)}} and {{countdistinct(false,distinctvals)}} (so
countdistinct indicates that in a distributed setup it requires distinctvals to
be returned by each shard)
* remove all the calcDistinct logic from {{StatsValuesFactory}} and instead
treat {{countdistinct}} and {{distinctvals}} as independently _requested_ stats
that have a dependent distributed computation relationship (just like "mean" vs
"sum / count")
* update the existing special syntactic sugar parsing for the top level
"calcdistinct" param to also include syntatic sugar parsing for the
calcdistinct local param - in both cases they become sugar for
{{countdistinct=true distinctvals=true}}
the end result being that users who want the {{countDistinct}} response info,
w/o the list of all {{distinctVals}} would just specify {{countdistinct=true}}
as a localparam (no other new top level params needed)
How does that sound?
these changes should already be fairly trivial to make if you want to take a
stab at a patch -- but there may be some patch collision in the code with the
work i'm doing in SOLR-6968. There will definitely be patch collisions in the
existing test cases, but once SOLR-6968 is commited the test changes should
actually be _much_ easier ... and i'm happy to try and tackle this as soon as
i'm done with that one (if nothing else, it will help me do a better apples to
apples benchmark of HLL vs countDistinct w/o the added distincatVals in the
final result)
> StatsComponent, calcdistinct, ability to disable distinctValues while keeping
> countDistinct
> -------------------------------------------------------------------------------------------
>
> Key: SOLR-7461
> URL: https://issues.apache.org/jira/browse/SOLR-7461
> Project: Solr
> Issue Type: Improvement
> Reporter: James Andres
> Labels: statscomponent
>
> When using calcdistinct with large amounts of data the distinctValues field
> can be extremely large. In cases where the countDistinct is only required it
> would be helpful if the server did not return distinctValues in the response.
> I'm no expert, but here are some ideas for how the syntax could look.
> {code}
> # Both countDistinct and distinctValues are returned, along with all other
> stats
> stats.calcdistinct=true&stats.field=myfield
> # Only countDistinct and distinctValues are returned
> stats.calcdistinct=true&stats.field={!countDistinct=true
> distinctValues=true}myfield
> # Only countDistinct is returned
> stats.calcdistinct=true&stats.field={!countDistinct=true}myfield
> # Only distinctValues is returned
> stats.calcdistinct=true&stats.field={!distinctValues=true}myfield
> {code}
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