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From Adrien Grand <jpou...@gmail.com>
Subject Re: New Lucene features and Solr indexes
Date Thu, 14 Feb 2013 15:26:54 GMT
On Wed, Feb 13, 2013 at 4:18 PM, Shawn Heisey <solr@elyograg.org> wrote:
> On 2/13/2013 2:42 AM, Adrien Grand wrote:
>> Doc values are like FieldCache except that you don't need to uninvert
>> values from the inverted index whenever you open a new Reader. I think
>> there are two reasons why you would like to turn doc values on:
>
>
> Confession -- that's almost gibberish to me!  At my current level of
> understanding, the pieces make some semblance of sense, but the whole thing
> falls apart before my head grasps it.  My fault, not yours. :)

What it means is that doc values achieve the same goal as the field
cache (the ability to quickly access the value of a given field for
any document) except that the hard work is done at indexing time
rather than whenever a new IndexReader is open.  This is generally a
better trade-off and I think the field cache is eventually going to be
deprecated or even removed (5.0 maybe?).

> This suggests that adding docvalues to the uniqueKey field would be a good
> idea for distributed searching in general, since the first phase of a
> distributed search only retrieves that field and score.  That assumes of
> course that the docvalues are fully utilized for retrieving fields during
> that initial phase.

Right, this would likely improve performance given than doc values
(even if disk-based) are more likely to be in memory than stored
fields. Another (better?) approach would be to use the internal Lucene
doc IDs for distributed search (I assumed there was an open JIRA issue
to do that but I can't find it).

-- 
Adrien

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