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https://issues.apache.org/jira/browse/LUCENE-5693?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14005740#comment-14005740
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Michael McCandless commented on LUCENE-5693:
--------------------------------------------

bq. couldn't we change the code to not send deleted docs' fields to the 
DocValues API too?

We could, but Rob is strongly against that, so I'll remove that TODO and make 
it clear this is just about not wasting IO/CPU writing deleted postings.

Also, remember that postings is the most costly part of the merge, so not 
writing deleted docs there gets us the most gains.

> don't write deleted documents on flush
> --------------------------------------
>
>                 Key: LUCENE-5693
>                 URL: https://issues.apache.org/jira/browse/LUCENE-5693
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Michael McCandless
>         Attachments: LUCENE-5693.patch
>
>
> When we flush a new segment, sometimes some documents are "born deleted", 
> e.g. if the app did a IW.deleteDocuments that matched some not-yet-flushed 
> documents.
> We already compute the liveDocs on flush, but then we continue (wastefully) 
> to send those known-deleted documents to all Codec parts.
> I started to implement this on LUCENE-5675 but it was too controversial.
> Also, I expect typically the number of deleted docs is 0, or small, so not 
> writing "born deleted" docs won't be much of a win for most apps.  Still it 
> seems silly to write them, consuming IO/CPU in the process, only to consume 
> more IO/CPU later for merging to re-delete them.



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