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From "Michael Busch (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1231) Column-stride fields (aka per-document Payloads)
Date Wed, 08 Apr 2009 03:49:12 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1231?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12696873#action_12696873
] 

Michael Busch commented on LUCENE-1231:
---------------------------------------

{quote}
is for column-stride fields to be a possible source for IndexReader.document() (or whatever
it becomes). Ie, when possible, that method should maybe pull from CSFs for values.
{quote}

Hmm, I'm not sure about this. The Lucene store is optimized for the case when ALL stored fields
for a document need to be loaded. E.g. to render search results you usually need to load multiple
fields. Then only one disk seek is needed to seek to the right document. Loading the fields
is then just one sequential I/O. 

If you e.g. want to show 5 fields per search result, you need to perform 1 seek per doc using
stored fields, but will need 5 seeks with CSFs.

My general rule of thumb is to use payloads/CSFs for data you need during hit collection (e.g.
a UID field or e.g. the norms, which is essential a CSF with fixed size), but ordinary stored
fields to load data for rendering search results.

> Column-stride fields (aka per-document Payloads)
> ------------------------------------------------
>
>                 Key: LUCENE-1231
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1231
>             Project: Lucene - Java
>          Issue Type: New Feature
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: 3.0
>
>
> This new feature has been proposed and discussed here:
> http://markmail.org/search/?q=per-document+payloads#query:per-document%20payloads+page:1+mid:jq4g5myhlvidw3oc+state:results
> Currently it is possible in Lucene to store data as stored fields or as payloads.
> Stored fields provide good performance if you want to load all fields for one
> document, because this is an sequential I/O operation.
> If you however want to load the data from one field for a large number of 
> documents, then stored fields perform quite badly, because lot's of I/O seeks 
> might have to be performed. 
> A better way to do this is using payloads. By creating a "special" posting list
> that has one posting with payload for each document you can "simulate" a column-
> stride field. The performance is significantly better compared to stored fields,
> however still not optimal. The reason is that for each document the freq value,
> which is in this particular case always 1, has to be decoded, also one position
> value, which is always 0, has to be loaded.
> As a solution we want to add real column-stride fields to Lucene. A possible
> format for the new data structure could look like this (CSD stands for column-
> stride data, once we decide for a final name for this feature we can change 
> this):
> CSDList --> FixedLengthList | <VariableLengthList, SkipList> 
> FixedLengthList --> <Payload>^SegSize 
> VariableLengthList --> <DocDelta, PayloadLength?, Payload> 
> Payload --> Byte^PayloadLength 
> PayloadLength --> VInt 
> SkipList --> see frq.file
> We distinguish here between the fixed length and the variable length cases. To
> allow flexibility, Lucene could automatically pick the "right" data structure. 
> This could work like this: When the DocumentsWriter writes a segment it checks 
> whether all values of a field have the same length. If yes, it stores them as 
> FixedLengthList, if not, then as VariableLengthList. When the SegmentMerger 
> merges two or more segments it checks if all segments have a FixedLengthList 
> with the same length for a column-stride field. If not, it writes a 
> VariableLengthList to the new segment. 
> Once this feature is implemented, we should think about making the column-
> stride fields updateable, similar to the norms. This will be a very powerful
> feature that can for example be used for low-latency tagging of documents.
> Other use cases:
> - replace norms
> - allow to store boost values separately from norms
> - as input for the FieldCache, thus providing significantly improved loading
> performance (see LUCENE-831)
> Things that need to be done here:
> - decide for a name for this feature :) - I think "column-stride fields" was
> liked better than "per-document payloads"
> - Design an API for this feature. We should keep in mind here that these 
> fields are supposed to be updateable.
> - Define datastructures.
> I would like to get this feature into 2.4. Feedback about the open questions
> is very welcome so that we can finalize the design soon and start 
> implementing.

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