Uwe Schindler created LUCENE-4548:
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Summary: BooleanFilter should optionally pass down further
restricted acceptDocs in the MUST case (and acceptDocs in general)
Key: LUCENE-4548
URL: https://issues.apache.org/jira/browse/LUCENE-4548
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 easily.
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 deletion-specific)!
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 does).
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