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From Adrien Grand <>
Subject Re: Stored fields and OS file caching
Date Fri, 04 Apr 2014 22:50:53 GMT
Hi Vitaly,

Doc values are indeed well-suited for grouping and sorting. However
stored fields remain better at returning field values to users since
they guarantee a worst-case of one disk seek per document.

The filesystem cache typically caches data by blocks of 4KB. This
plays more nicely with doc values: given that they are stored in a
column-stride fashion, you are load only those field values into the
filesystem cache. On the other hand with stored fields, data is stored
sequentially in a very large file, so whenever you read a single field
value, the filesystem cache would load a 4KB block of data into the
filesystem cache that likely contains other fields' values that you
are not interested in.

On Sat, Apr 5, 2014 at 12:23 AM, Vitaly Funstein <> wrote:
> I use stored fields to load values for the following use cases:
> - to return per-document values as is, requested by the user - similar to
> listing DB columns you are interested in, in a "select ..." clause.
> - to perform aggregate function calculations while forming the result set
> (if requested).
> - for group-by type queries (would like to switch to the native grouping
> API, but don't think it supports grouping on multiple fields, or aggregate
> functions).
> - and finally, as I mentioned - to sort search results, also when requested.
> Evidently, even for simple queries that don't require any of the
> post-processing above but ask for a set of values from each document,
> there's still non-trivial amount of disk activity... hence, I started
> second-guessing the implementation.
> On Fri, Apr 4, 2014 at 3:00 PM, Uwe Schindler <> wrote:
>> Hi,
>> What are you doing with the stored fields? They are not deprecated and
>> also not really slow, unless you scan over millions of documents in random
>> access order. To display serach results, DocValues are of no use.
>> Uwe
>> -----
>> Uwe Schindler
>> H.-H.-Meier-Allee 63, D-28213 Bremen
>> eMail:
>> > -----Original Message-----
>> > From: Vitaly Funstein []
>> > Sent: Friday, April 04, 2014 9:44 PM
>> > To:
>> > Subject: Stored fields and OS file caching
>> >
>> > I have heard here that stored fields don't work well with OS file
>> caching.
>> > Could someone elaborate on why that is? I am using Lucene 4.6 and we do
>> > use stored fields but not doc values; it appears most of the benefit
>> from the
>> > latter comes as improvement in sorting performance, and I don't actually
>> use
>> > Lucene for sorting at all; rather, it's done on a post-processing basis,
>> based on
>> > stored field values (in a nutshell, the reason for this is Lucene's
>> inability to tell
>> > apart terms that are empty strings vs. a missing value, resulting in
>> unstable
>> > sort order on such fields).
>> >
>> > I am not sure if switching to using doc values fields from stored fields
>> entirely
>> > would help leverage OS file cache better... what worries me is that when
>> > processing queries requesting multiple values from the document, doc
>> value
>> > fields could cause multiple disk seeks to fetch values for each field, as
>> > opposed to just one with stored fields.
>> >
>> > Am I way off in my understanding of how this works? Any guidelines, as
>> > general as they may be, are appreciated.
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