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https://issues.apache.org/jira/browse/LUCENE-5441?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14180023#comment-14180023
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Robert Muir commented on LUCENE-5441:
-------------------------------------
{quote}
Just one general question: Is it really needed that the iterator also have
cost()? In my opinion, it should be fine when you call cost() on the DocIdSet.
If you already have an iterator, why call cost - it returns the same as the
DocIdSet (in general)? This would make the extra ctor parameter for the
FixedBitSetIterator obsolete.
{quote}
Currently, cost() is defined on DocumentIDSetIterator and of course subclasses:
docsenum & co: implemented as docFreq by postings lists, e.g. termscorer as its
docsEnum.cost().
This is used by conjunctionscorer/minshouldmatch/filteredquery etc to do
conjunctions and so on.
> Decouple DocIdSet from OpenBitSet and FixedBitSet
> -------------------------------------------------
>
> Key: LUCENE-5441
> URL: https://issues.apache.org/jira/browse/LUCENE-5441
> Project: Lucene - Core
> Issue Type: Task
> Components: core/other
> Affects Versions: 4.6.1
> Reporter: Uwe Schindler
> Fix For: Trunk
>
> Attachments: LUCENE-5441.patch, LUCENE-5441.patch, LUCENE-5441.patch,
> LUCENE-5441.patch, LUCENE-5441.patch
>
>
> Back from the times of Lucene 2.4 when DocIdSet was introduced, we somehow
> kept the stupid "filters can return a BitSet directly" in the code. So lots
> of Filters return just FixedBitSet, because this is the superclass (ideally
> interface) of FixedBitSet.
> We should decouple that and *not* implement that abstract interface directly
> by FixedBitSet. This leads to bugs e.g. in BlockJoin, because it used Filters
> in a wrong way, just because it was always returning Bitsets. But some
> filters actually don't do this.
> I propose to let FixedBitSet (only in trunk, because that a major backwards
> break) just have a method {{asDocIdSet()}}, that returns an anonymous
> instance of DocIdSet: bits() returns the FixedBitSet itsself, iterator()
> returns a new Iterator (like it always did) and the cost/cacheable methods
> return static values.
> Filters in trunk would need to be changed like that:
> {code:java}
> FixedBitSet bits = ....
> ...
> return bits;
> {code}
> gets:
> {code:java}
> FixedBitSet bits = ....
> ...
> return bits.asDocIdSet();
> {code}
> As this methods returns an anonymous DocIdSet, calling code can no longer
> rely or check if the implementation behind is a FixedBitSet.
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