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From "Toke Eskildsen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2335) optimization: when sorting by field, if index has one segment and field values are not needed, do not load String[] into field cache
Date Fri, 26 Mar 2010 15:16:31 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2335?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12850174#action_12850174
] 

Toke Eskildsen commented on LUCENE-2335:
----------------------------------------

The exposed DirectoryReader is now implemented and the timing does not look horrible. Stepping
through ordered terms for a DirectoryReader to build the structure needed for sorting is a
bit faster than the sum of sorting the individual segments. That's not quite good enough,
but there is still room for a clever read-ahead cache to make iteration more sequential at
the SegmentReader-level.

To give a ball-park figure: Something like 1 minute / 1 million terms for segment-level sorting,
which is re-used for non-changed segments on re-open. Then something like half that time for
directory-level merging, which must be done fully at re-open.

There's no easy to use plug-in replacement yet (and it seems hard to do anyway, as the sorter
gets the readers one at a time) and the code at github is in shambles, so no patch either.
Sorry. I expect to get back to hacking in a week.

> 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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