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https://issues.apache.org/jira/browse/LUCENE-584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12515494
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Mark Harwood commented on LUCENE-584:
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Thanks for the reply, Paul.
I saw BitSetMatcher etc and appreciate the motivation behind the design for
alternative implementations . What concerns me with the Matcher API in general
is that Matchers have non-threadsafe safe state (i.e. the current position
required to support next() )and as such aren't safely cachable in the same way
as BitSets. I see the searcher code uses the safer skipTo() rather than next()
but there's still the "if(exhausted)" thread safety problem to worry about
which is why I raised points 1 and 4.
Additionally, combining Bitsets using Booolean logic is one method call whereas
combining heterogenous Matchers using Boolean logic requires iteration across
them and therefore potentially many method calls (point 3). I haven't
benchmarked this but I imagine it to be significantly slower?
I use BooleanFilter a lot for security where many large sets are cached and
combined on the fly - caching all the possible combinations as single bitsets
would lead to too many possible combinations.
Cheers
Mark
> Decouple Filter from BitSet
> ---------------------------
>
> Key: LUCENE-584
> URL: https://issues.apache.org/jira/browse/LUCENE-584
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Search
> Affects Versions: 2.0.1
> Reporter: Peter Schäfer
> Priority: Minor
> Attachments: bench-diff.txt, bench-diff.txt,
> Matcher-core20070725.patch, Matcher-default20070725.patch,
> Matcher-ground20070725.patch, Some Matchers.zip
>
>
> {code}
> package org.apache.lucene.search;
> public abstract class Filter implements java.io.Serializable
> {
> public abstract AbstractBitSet bits(IndexReader reader) throws IOException;
> }
> public interface AbstractBitSet
> {
> public boolean get(int index);
> }
> {code}
> It would be useful if the method =Filter.bits()= returned an abstract
> interface, instead of =java.util.BitSet=.
> Use case: there is a very large index, and, depending on the user's
> privileges, only a small portion of the index is actually visible.
> Sparsely populated =java.util.BitSet=s are not efficient and waste lots of
> memory. It would be desirable to have an alternative BitSet implementation
> with smaller memory footprint.
> Though it _is_ possibly to derive classes from =java.util.BitSet=, it was
> obviously not designed for that purpose.
> That's why I propose to use an interface instead. The default implementation
> could still delegate to =java.util.BitSet=.
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