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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: Document ID shuffling under 2.3.x (on merge?)
Date Mon, 17 Mar 2008 08:38:46 GMT

Daniel Noll wrote:

> On Thursday 13 March 2008 19:46:20 Michael McCandless wrote:
>> But, when a normal merge of segments with deletions completes, your
>> docIDs will shift.  In trunk we now explicitly compute the docID
>> shifting that happens after a merge, because we don't always flush
>> pending deletes when flushing added docs, but this is all done
>> privately to IndexWriter.
>
> I don't need to worry about deleted documents as such things don't  
> exist in
> our system, hence the optimisation based on document IDs.
>
>> I'm a little confused: you said optimize() introduces the problem,
>> but, it sounds like optimize() should be fixing the problem because
>> it compacts all docIDs to match what you were "guessing" outside of
>> Lucene?  Can you post the full stack trace of the exceptions you're
>> hitting?
>
> You're misunderstanding how we're getting the ID, that's all.   
> We're getting
> it by calling docCount() (after adding) and subtracting 1, which is
> guaranteed to give the right ID at the time of indexing, although  
> of course,
> later is another matter entirely.  We were operating from now out  
> of date
> information which says the IDs don't shift unless you call delete...
>
> Example:
>
>   add document, assume ID 0 (docCount = 1)
>   add document, assume ID 1 (docCount = 2)
>   add document, FAILS - assumed not added
>   re-add document minus reader fields, assume ID 3 (docCount = 4)
>
> So the ID assumptions are correct at this point; when optimize() is  
> called, it
> shifts the third document sucht that it then has ID 2, and our  
> internal
> counts become wrong.

OK now I understand: docCount() is correct at the time, but then when  
a merge or optimize merges a segment that has a document that hit an  
exception, the IDs shift.

> I've backported the expungeDeleted() patch into 2.3 and will be  
> testing it out
> next; seems it will do more or less what we want and merging the  
> deleted
> document should be relatively quick as it will only ever exist in  
> the in
> DocumentsWriter's in-memory buffer.

Well ... expungeDeletes() first forces a flush, at which point the  
deletions are flushed as a .del file against the just flushed  
segment.  Still, if you call expungeDeletes after every flush  
(commit) then it's only 1 segment whose deletions need to be expunged  
so it should be fast.

Mike

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