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From "Andrzej Bialecki (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3837) A modest proposal for updateable fields
Date Thu, 01 Mar 2012 20:17:57 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3837?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13220315#comment-13220315
] 

Andrzej Bialecki  commented on LUCENE-3837:
-------------------------------------------

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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