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https://issues.apache.org/jira/browse/LUCENE-1526?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12666626#action_12666626
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Michael McCandless commented on LUCENE-1526:
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In addition to deletions, we also eventually need an
incremental-copy-on-write data structure for norms. Full
copy-on-write for norms (which LUCENE-1314 will enable) is even more
costly than full copy-on-write for deletions since each norm is 1 byte
vs 1 bit for deletions.
Though, it seems less common the people are setting norms, so it's
probably lower priority.
But I can still see "real-time norm changing" as being quite useful,
eg if you want to bias normal relevance sorting to mixin some measure
of popularity (eg recording how often each document is clicked), it'd
be good to have real-time norm updating for that.
> Tombstone deletions in IndexReader
> ----------------------------------
>
> Key: LUCENE-1526
> URL: https://issues.apache.org/jira/browse/LUCENE-1526
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Affects Versions: 2.4
> Reporter: Jason Rutherglen
> Priority: Minor
> Fix For: 2.9
>
> Original Estimate: 168h
> Remaining Estimate: 168h
>
> SegmentReader currently uses a BitVector to represent deleted docs.
> When performing rapid clone (see LUCENE-1314) and delete operations,
> performing a copy on write of the BitVector can become costly because
> the entire underlying byte array must be created and copied. A way to
> make this clone delete process faster is to implement tombstones, a
> term coined by Marvin Humphrey. Tombstones represent new deletions
> plus the incremental deletions from previously reopened readers in
> the current reader.
> The proposed implementation of tombstones is to accumulate deletions
> into an int array represented as a DocIdSet. With LUCENE-1476,
> SegmentTermDocs iterates over deleted docs using a DocIdSet rather
> than accessing the BitVector by calling get. This allows a BitVector
> and a set of tombstones to by ANDed together as the current reader's
> delete docs.
> A tombstone merge policy needs to be defined to determine when to
> merge tombstone DocIdSets into a new deleted docs BitVector as too
> many tombstones would eventually be detrimental to performance. A
> probable implementation will merge tombstones based on the number of
> tombstones and the total number of documents in the tombstones. The
> merge policy may be set in the clone/reopen methods or on the
> IndexReader.
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