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https://issues.apache.org/jira/browse/LUCENE-3837?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13220315#comment-13220315
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Andrzej Bialecki commented on LUCENE-3837:
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bq. Could we use the actual docID (ie same docID as the base segment)?
Updates may arrive out of order, so the updates will naturally get different
internal IDs (also, if you wanted to use the same ids they would have gaps). I
don't know if various parts of Lucene can handle out of order ids coming from
iterators? If we wanted to match the ids early then we would have to sort them,
a la IndexSorter, on every flush and on every merge, which seems too costly.
So, a re-mapping structure seems like a decent compromise. Yes, it could be
large - we could put artificial limits on the number of updates before we do a
merge.
bq. Also, can't we directly write the stacked segments ourselves? (Ie, within a
single IW).
I don't know, it didn't seem likely to me - AFAIK IW operates on a single
segment before flushing it? And updates could refer to docs outside the current
segment.
> A modest proposal for updateable fields
> ---------------------------------------
>
> Key: LUCENE-3837
> URL: https://issues.apache.org/jira/browse/LUCENE-3837
> Project: Lucene - Java
> Issue Type: New Feature
> Components: core/index
> Affects Versions: 4.0
> Reporter: Andrzej Bialecki
>
> I'd like to propose a simple design for implementing updateable fields in
> Lucene. This design has some limitations, so I'm not claiming it will be
> appropriate for every use case, and it's obvious it has some performance
> consequences, but at least it's a start...
> This proposal uses a concept of "overlays" or "stacked updates", where the
> original data is not removed but instead it's overlaid with the new data. I
> propose to reuse as much of the existing APIs as possible, and represent
> updates as an IndexReader. Updates to documents in a specific segment would
> be collected in an "overlay" index specific to that segment, i.e. there would
> be as many overlay indexes as there are segments in the primary index.
> A field update would be represented as a new document in the overlay index .
> The document would consist of just the updated fields, plus a field that
> records the id in the primary segment of the document affected by the update.
> These updates would be processed as usual via secondary IndexWriter-s, as
> many as there are primary segments, so the same analysis chains would be
> used, the same field types, etc.
> On opening a segment with updates the SegmentReader (see also LUCENE-3836)
> would check for the presence of the "overlay" index, and if so it would open
> it first (as an AtomicReader? or it would open individual codec format
> readers? perhaps it should load the whole thing into memory?), and it would
> construct an in-memory map between the primary's docId-s and the overlay's
> docId-s. And finally it would wrap the original format readers with "overlay
> readers", initialized also with the id map.
> Now, when consumers of the 4D API would ask for specific data, the "overlay
> readers" would first re-map the primary's docId to the overlay's docId, and
> check whether overlay data exists for that docId and this type of data (e.g.
> postings, stored fields, vectors) and return this data instead of the
> original. Otherwise they would return the original data.
> One obvious performance issue with this appraoch is that the sequential
> access to primary data would translate into random access to the overlay
> data. This could be solved by sorting the overlay index so that at least the
> overlay ids increase monotonically as primary ids do.
> Updates to the primary index would be handled as usual, i.e. segment merges,
> since the segments with updates would pretend to have no overlays) would just
> work as usual, only the overlay index would have to be deleted once the
> primary segment is deleted after merge.
> Updates to the existing documents that already had some fields updated would
> be again handled as usual, only underneath they would open an IndexWriter on
> the overlay index for a specific segment.
> That's the broad idea. Feel free to pipe in - I started some coding at the
> codec level but got stuck using the approach in LUCENE-3836. The approach
> that uses a modified SegmentReader seems more promising.
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