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https://issues.apache.org/jira/browse/LUCENE-2335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12847650#action_12847650
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Toke Eskildsen commented on LUCENE-2335:
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I obviously have a share in this and would like to give the inner workings a go 
(I've made a proof of concept at http://github.com/tokee/lucene that I'll use 
as base). However, I am not all that familiar with the finer details of the 
Lucene code base, so I'll proably need help for integrating the code.

If I read the Lucene code correctly, the actual Strings need only be resolved 
for the requested X documents (using FieldComparator.value). If this is true, 
there is no need for stored fields: We can get the Strings from the indexed 
fields instead, as we keep track of the ordinals for the Terms.

> optimization: when sorting by field, if index has one segment and field 
> values are not needed, do not load String[] into field cache
> ------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-2335
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2335
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>            Reporter: Michael McCandless
>            Priority: Minor
>             Fix For: 3.1
>
>
> Spinoff from java-dev thread "Sorting with little memory: A suggestion", 
> started by Toke Eskildsen.
> When sorting by SortField.STRING we currently ask FieldCache for a 
> StringIndex on that field.
> This can consumes tons of RAM, when the values are mostly unique (eg a title 
> field), as it populates both int[] ords as well as String[] values.
> But, if the index is only one segment, and the search sets fillFields=false, 
> we don't need the String[] values, just the int[] ords.  If the app needs to 
> show the fields it can pull them (for the 1 page) from stored fields.
> This can be a potent optimization -- alot of RAM saved -- for optimized 
> indexes.
> When fixing this we must take care to share the int[] ords if some queries do 
> fillFields=true and some =false... ie, FieldCache will be called twice and it 
> should share the int[] ords across those invocations.

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