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From "David Smiley (JIRA)" <>
Subject [jira] [Commented] (LUCENE-4548) BooleanFilter should optionally pass down further restricted acceptDocs in the MUST case (and acceptDocs in general)
Date Thu, 08 Nov 2012 20:00:12 GMT


David Smiley commented on LUCENE-4548:

My broad comments on this having looked at a variety of these classes, is that the whole situation
is very confusing.  There are a bunch of classes here related to filtering that if you consider
the sum total of them, it seems like a bit much to get a handle on: Filter, ChainedFilter,
BooleanFilter, FilteredQuery, FilteredDocIdSet, BitsFilteredDocIdSet.  I'm probably missing
some.  And then of course Filter != Query but sometimes they need to be adapted to each other.
 I bet there are a dozen ways I could skin this cat (?).  That's a problem.

Looking ahead at Lucene 5, can we think of a smaller set of classes for filters and chaining
them (with AND, OR, ...), and annotating which are "expensive" and considering random access
vs doc id iteration?  The title of this issue seems like it's a band-aid to the API complexity
that probably makes it worse.
> BooleanFilter should optionally pass down further restricted acceptDocs in the MUST case
(and acceptDocs in general)
> --------------------------------------------------------------------------------------------------------------------
>                 Key: LUCENE-4548
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Uwe Schindler
> Spin-off from dev@lao:
> {quote}
> bq. I am about to write a Filter that only operates on a set of documents that have already
passed other filter(s).  It's rather expensive, since it has to use DocValues to examine a
value and then determine if its a match.  So it scales O(n) where n is the number of documents
it must see.  The 2nd arg of getDocIdSet is Bits acceptDocs.  Unfortunately Bits doesn't have
an int iterator but I can deal with that seeing if it extends DocIdSet.
> bq. I'm looking at BooleanFilter which I want to use and I notice that it passes null
to filter.getDocIdSet for acceptDocs, and it justifies this with the following comment:
> bq. // we dont pass acceptDocs, we will filter at the end using an additional filter
> the idea of passing the already build bits for the MUST is a good idea and can be implemented
> The reason why the acceptDocs were not passed down is the new way of filter works in
Lucene 4.0 and to optimize caching. Because accept docs are the only thing that changes when
deletions are applied and filters are required to handle them separately:  whenever something
is able to cache (e.g. CachingWrapperFilter), the acceptDocs are not cached, so the underlying
filters get a null acceptDocs to produce the full bitset and the filtering is done when CachingWrapperFilter
gets the “uptodate” acceptDocs. But for this case this does not matter if the first filter
clause does not get acceptdocs, but later MUST clauses of course can get them (they are not
> Can you open issue to optimize the MUST case (possibly MUST_NOT, too)?
> Another thing that could help here: You can stop using BooleanFilter if you can apply
the filters sequentially (only MUST clauses) by wrapping with multiple FilteredQuery: new
FilteredQuery(new FilteredQuery(originalQuery, clause1), clause2). If the DocIdSets enable
bits() and the FilteredQuery autodetection decides to use random access filters, the acceptdocs
are also passed down from the outside to the inner, removing the documents filtered out.
> {quote}
> Maybe BooleanFilter should have 2 modes (Boolean ctor argument): Passing down the acceptDocs
to every filter (for the case where Filter calculation is expensive and accept docs help to
limit the calculations) or not passing down (if the filter is cheap and the multiple acceptDocs
bit checks for every single filter is more expensive – which is then more effective, e.g.
when the Filter is only a cached bitset). The first mode would also optimize the MUST/MUST_NOT
case to pass down the further restricted acceptDocs on later filters (just like FilteredQuery

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