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 <vfunst...@gmail.com> 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 <u...@thetaphi.de> 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 >> http://www.thetaphi.de >> eMail: u...@thetaphi.de >> >> >> > -----Original Message----- >> > From: Vitaly Funstein [mailto:vfunst...@gmail.com] >> > Sent: Friday, April 04, 2014 9:44 PM >> > To: java-user@lucene.apache.org >> > 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. >> >> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: java-user-unsubscr...@lucene.apache.org >> For additional commands, e-mail: java-user-h...@lucene.apache.org >> >> -- Adrien --------------------------------------------------------------------- To unsubscribe, e-mail: java-user-unsubscr...@lucene.apache.org For additional commands, e-mail: java-user-h...@lucene.apache.org
