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From "Uwe Schindler (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1483) Change IndexSearcher multisegment searches to search each individual segment using a single HitCollector
Date Wed, 27 Oct 2010 15:18:23 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1483?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12925413#action_12925413
] 

Uwe Schindler commented on LUCENE-1483:
---------------------------------------

Reverted the removal of SorterTemplate, its now replaced by a better implementation, only
inspired by CGLIB: LUCENE-2719

> Change IndexSearcher multisegment searches to search each individual segment using a
single HitCollector
> --------------------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-1483
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1483
>             Project: Lucene - Java
>          Issue Type: Improvement
>    Affects Versions: 2.9
>            Reporter: Mark Miller
>            Assignee: Michael McCandless
>            Priority: Minor
>             Fix For: 2.9
>
>         Attachments: LUCENE-1483-backcompat.patch, LUCENE-1483-partial.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch,
LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, LUCENE-1483.patch, sortBench.py,
sortCollate.py
>
>
> This issue changes how an IndexSearcher searches over multiple segments. The current
method of searching multiple segments is to use a MultiSegmentReader and treat all of the
segments as one. This causes filters and FieldCaches to be keyed to the MultiReader and makes
reopen expensive. If only a few segments change, the FieldCache is still loaded for all of
them.
> This patch changes things by searching each individual segment one at a time, but sharing
the HitCollector used across each segment. This allows FieldCaches and Filters to be keyed
on individual SegmentReaders, making reopen much cheaper. FieldCache loading over multiple
segments can be much faster as well - with the old method, all unique terms for every segment
is enumerated against each segment - because of the likely logarithmic change in terms per
segment, this can be very wasteful. Searching individual segments avoids this cost. The term/document
statistics from the multireader are used to score results for each segment.
> When sorting, its more difficult to use a single HitCollector for each sub searcher.
Ordinals are not comparable across segments. To account for this, a new field sort enabled
HitCollector is introduced that is able to collect and sort across segments (because of its
ability to compare ordinals across segments). This TopFieldCollector class will collect the
values/ordinals for a given segment, and upon moving to the next segment, translate any ordinals/values
so that they can be compared against the values for the new segment. This is done lazily.
> All and all, the switch seems to provide numerous performance benefits, in both sorted
and non sorted search. We were seeing a good loss on indices with lots of segments (1000?)
and certain queue sizes / queries, but the latest results seem to show thats been mostly taken
care of (you shouldnt be using such a large queue on such a segmented index anyway).
> * Introduces
> ** MultiReaderHitCollector - a HitCollector that can collect across multiple IndexReaders.
Old HitCollectors are wrapped to support multiple IndexReaders.
> ** TopFieldCollector - a HitCollector that can compare values/ordinals across IndexReaders
and sort on fields.
> ** FieldValueHitQueue - a Priority queue that is part of the TopFieldCollector implementation.
> ** FieldComparator - a new Comparator class that works across IndexReaders. Part of the
TopFieldCollector implementation.
> ** FieldComparatorSource - new class to allow for custom Comparators.
> * Alters
> ** IndexSearcher uses a single HitCollector to collect hits against each individual SegmentReader.
All the other changes stem from this ;)
> * Deprecates
> ** TopFieldDocCollector
> ** FieldSortedHitQueue

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