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https://issues.apache.org/jira/browse/LUCENE-1076?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12733819#action_12733819
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Michael McCandless commented on LUCENE-1076:
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bq. But how is a new doc ID allocated?
Doc IDs are logically assigned by summing docCount of all segments before me,
as my base, and then adding to the "index" of the doc within my segment. Ie,
the base of a given segment is not stored anywhere, so we are always free to
shuffle up the order of segments and nothing in Lucene should care (but, the
app might).
> 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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