First cut at column-stride fields (index values storage)
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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: Michael McCandless
Fix For: 3.1
Attachments: LUCENE-2186.patch
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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