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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2186) First cut at column-stride fields (index values storage)
Date Wed, 24 Nov 2010 18:31:15 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2186?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12935428#action_12935428
] 

Michael McCandless commented on LUCENE-2186:
--------------------------------------------

I think this is very close!!

  * Using attr source as the way to specify the docValue is nice in
    that we get full extensibility, but, it's also heavyweight
    compared to a dedicated API (ie, .setIntValue, etc.).  So I think
    this means apps that use doc values really must re-use their Field
    instances (if they are using doc values) else indexing performance
    will likely take a good hit.

  * ValuesField is nice sugar on top (of the attr) :) Can you add some
    jdocs to ValuesField? EG it's not stored/indexed.  It's OK to have
    same field name as existing field (hmm... is it)?  Etc.

  * Did you want to make FieldsConsumer.addValuesField abstract?

  * The javadoc above DocValues.Source is wrong -- Source is not just
    for ints.

  * You can change jdocs like "This feature is experimental and the
    API is free to change in non-backwards-compatible ways." to
    @lucene.experimental :)  (eg in Values.java)

  * So, you're not allowed to change the DocValues type for a field
    once you've set it the first time... and, also, segments cannot be
    merged if the same field has different value types.  I'm thinking
    it's really important now to carry over the same FieldInfos from
    the last segment when opening the writer (LUCENE-1737)... because
    hitting that IllegalStateExc during merge is a trap.  This would
    let us change that IllegalStateExc into an assert (in
    SegmentMerger) and also turn the assert back on in FieldsConsumer.

  * Should we rename MissingValues to MissingValue? Ie it holds the
    single value for your type that represents "missing"?

  * We need better names than PagedBytes.fillUsingLengthPrefix,2,3,4
    heh.

  * It'd be nice to have a more approachable test case that shows the
    "simple" way to index doc values, ie using ValuesField instead of
    getting the attr, getting the intsRef, setting it, etc.  I think
    such an "example" should be very compact right?


> First cut at column-stride fields (index values storage)
> --------------------------------------------------------
>
>                 Key: LUCENE-2186
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2186
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: Index
>            Reporter: Michael McCandless
>            Assignee: Simon Willnauer
>             Fix For: CSF branch, 4.0
>
>         Attachments: LUCENE-2186.patch, LUCENE-2186.patch, LUCENE-2186.patch, LUCENE-2186.patch,
LUCENE-2186.patch, mem.py
>
>
> I created an initial basic impl for storing "index values" (ie
> column-stride value storage).  This is still a work in progress... but
> the approach looks compelling.  I'm posting my current status/patch
> here to get feedback/iterate, etc.
> The code is standalone now, and lives under new package
> oal.index.values (plus some util changes, refactorings) -- I have yet
> to integrate into Lucene so eg you can mark that a given Field's value
> should be stored into the index values, sorting will use these values
> instead of field cache, etc.
> It handles 3 types of values:
>   * Six variants of byte[] per doc, all combinations of fixed vs
>     variable length, and stored either "straight" (good for eg a
>     "title" field), "deref" (good when many docs share the same value,
>     but you won't do any sorting) or "sorted".
>   * Integers (variable bit precision used as necessary, ie this can
>     store byte/short/int/long, and all precisions in between)
>   * Floats (4 or 8 byte precision)
> String fields are stored as the UTF8 byte[].  This patch adds a
> BytesRef, which does the same thing as flex's TermRef (we should merge
> them).
> This patch also adds basic initial impl of PackedInts (LUCENE-1990);
> we can swap that out if/when we get a better impl.
> This storage is dense (like field cache), so it's appropriate when the
> field occurs in all/most docs.  It's just like field cache, except the
> reading API is a get() method invocation, per document.
> Next step is to do basic integration with Lucene, and then compare
> sort performance of this vs field cache.
> For the "sort by String value" case, I think RAM usage & GC load of
> this index values API should be much better than field caache, since
> it does not create object per document (instead shares big long[] and
> byte[] across all docs), and because the values are stored in RAM as
> their UTF8 bytes.
> There are abstract Writer/Reader classes.  The current reader impls
> are entirely RAM resident (like field cache), but the API is (I think)
> agnostic, ie, one could make an MMAP impl instead.
> I think this is the first baby step towards LUCENE-1231.  Ie, it
> cannot yet update values, and the reading API is fully random-access
> by docID (like field cache), not like a posting list, though I
> do think we should add an iterator() api (to return flex's DocsEnum)
> -- eg I think this would be a good way to track avg doc/field length
> for BM25/lnu.ltc scoring.

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