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https://issues.apache.org/jira/browse/LUCENE-3606?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13158693#comment-13158693
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Michael McCandless commented on LUCENE-3606:
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I appreciate the purity of a truly read-only IndexReader, but there
are things deleting from IR can do that IW cannot:
* Real-time deletes: you can delete from IR and have it take
immediate effect on a searcher using that IR. With IW you have to
do an NRT reopen instead.
* Delete by docID... users seem to use/want this (eg ask from time
to time if we can do this in IW, which we can't since merging can
shift docIDs at any time).
* IR tells you how many documents were affected by a delete-by-Term.
IW cannot, since it buffers and only resolves all deletes in bulk
later.
Now... just because IR can do these things that IW cannot.... doesn't
mean they are compelling / important. Ie maybe no app out there
relies on these special things and so we can lose this functionality?
I'm not sure...
On removing IR.setNorm, I agree we should just remove it: in trunk now
you can make a custom sim that boost norms "live", so we won't lose
any functionality. The setNorm code (and the implications -- all the
per-field normGen, reference counting, scary SR.reopen, etc.) is truly
hairy; would be great to nuke it all.
> Make IndexReader really read-only in Lucene 4.0
> -----------------------------------------------
>
> Key: LUCENE-3606
> URL: https://issues.apache.org/jira/browse/LUCENE-3606
> Project: Lucene - Java
> Issue Type: Task
> Components: core/index
> Affects Versions: 4.0
> Reporter: Uwe Schindler
>
> As we change API completely in Lucene 4.0 we are also free to remove
> read-write access and commits from IndexReader. This code is so hairy and
> buggy (as investigated by Robert and Mike today) when you work on
> SegmentReader level but forget to flush in the DirectoryReader, so its better
> to really make IndexReaders readonly.
> Currently with IndexReader you can do things like:
> - delete/undelete Documents -> Can be done by with IndexWriter, too (using
> deleteByQuery)
> - change norms -> this is a bad idea in general, but when we remove norms at
> all and replace by DocValues this is obsolete already. Changing DocValues
> should also be done using IndexWriter in trunk (once it is ready)
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