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https://issues.apache.org/jira/browse/LUCENE-1076?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12734169#action_12734169
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Michael McCandless commented on LUCENE-1076:
--------------------------------------------

maxDoc() does reflect the number of docs in the index.  It's simply the sum of 
docCount for all segments.  Shuffling the order of the segments, or allowing 
non-contiguous segments to be merged, won't change how maxDoc() is computed.

New docIDs are allocating by incrementing an integer (starting with 0) for the 
buffered docs.  When a segment gets flushed, we reset that to 0.  Ie, docIDs 
are stored within one segment; they have no "context" from prior segments.

> 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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