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https://issues.apache.org/jira/browse/LUCENE-3837?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13220207#comment-13220207
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Michael McCandless commented on LUCENE-3837:
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Could we use the actual docID (ie same docID as the base segment)? This way we
wouldn't need the (possibly large) int[] to remap on each access. I guess for
postings this is OK (we can pass PostingsFormat any docIDs), but for eg stored
fields, term vectors, doc values, it's not (they can't handle "sparse" docIDs).
Also, can't we directly write the stacked segments ourselves? (Ie, within a
single IW).
We'd need to extend SegmentInfo(s) to record which segments stack on which, and
fix MP to understand stacking (and aggressively target the stacks).
> 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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