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https://issues.apache.org/jira/browse/LUCENE-1076?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12733778#action_12733778
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Shai Erera commented on LUCENE-1076:
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Well ... what I was thinking of is that even if the app does not care about
internal doc IDs, the Lucene code may very well care. If we don't shift doc IDs
back, it means maxDoc will continue to grow, and at some point (extreme case
though), maxDoc will equal 1M, while there will be just 50K docs in the index.
AFAIU, maxDoc is used today to determine array length in FieldCache, I've seen
it used in IndexSearcher to sort the sub readers (at least in the past) etc. So
perhaps alongside maxDoc we'll need to keep a curNumDocs member to track the
actual number of documents?
But I have a feeling this will also get complicated.
> Allow MergePolicy to select non-contiguous merges
> -------------------------------------------------
>
> Key: LUCENE-1076
> URL: https://issues.apache.org/jira/browse/LUCENE-1076
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Affects Versions: 2.3
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Priority: Minor
> Attachments: LUCENE-1076.patch
>
>
> I started work on this but with LUCENE-1044 I won't make much progress
> on it for a while, so I want to checkpoint my current state/patch.
> For backwards compatibility we must leave the default MergePolicy as
> selecting contiguous merges. This is necessary because some
> applications rely on "temporal monotonicity" of doc IDs, which means
> even though merges can re-number documents, the renumbering will
> always reflect the order in which the documents were added to the
> index.
> Still, for those apps that do not rely on this, we should offer a
> MergePolicy that is free to select the best merges regardless of
> whether they are continuguous. This requires fixing IndexWriter to
> accept such a merge, and, fixing LogMergePolicy to optionally allow
> it the freedom to do so.
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