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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 Mon, 09 Aug 2010 13:08:16 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2186?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12896530#action_12896530
] 

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

Great work Simon!

Now that FieldCache has cut over to shared byte[] blocks, them mem
gains for CSF when storing byte[] data are mostly gone.

But, there is still important benefits with CSF:

  * The full image is stored on-disk (= much faster than uninversion
    (& sometimes sorting) that FieldCache does, on startup)

  * You can specify all 6 combinations of variable/fixed length X
    straight/deref/sorted.  FieldCache is either var-length X deref
    (FieldCache.getTerms) or var-length X sorted
    (FieldCache.getTermsIndex).

  * It should be more extensible, ie, you can make your own attrs to
    store whatever you want.  EG we should be able to use this to
    store the flex scoring stats (LUCENE-2392).

The end-user API is rather cumbersome now (ie, that the user must
interact directly w/ attrs).  It seems like we should have a sugar
layer on top, eg an IntField(Type) and I can do IntField.set/get.

Also... maybe we should use Attrs the way NumericField does.  Ie, for
CSF we'd have a TokenStream (single valued, for now anyway), and then
attrs could be added to it.  If we can get attr serialization
(LUCENE-2125) online, then we can refactor all the read/write code in
this issue as the default attr serializers?  And, then, indexer would
have no special code for CSF in particular.  It just asks attrs to
serialize themselves...

Shouldn't FloatsRef be FloatRef (same for IntsRef)?  It's ref'ing a
single value 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: 4.0
>
>         Attachments: 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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