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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-1372) Proposal: introduce more sensible sorting when a doc has multiple values for a term
Date Fri, 27 Mar 2009 21:09:50 GMT


Michael McCandless commented on LUCENE-1372:

The goals being discussed under LUCENE-831 should match the first proposed solution, by giving
you custom control over how the FieldCache array is computed from the un-inverted terms.

> Proposal: introduce more sensible sorting when a doc has multiple values for a term
> -----------------------------------------------------------------------------------
>                 Key: LUCENE-1372
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>    Affects Versions: 2.3.2
>            Reporter: Paul Cowan
>            Priority: Minor
>         Attachments: LUCENE-1372-MultiValueSorters.patch, lucene-multisort.patch
> At the moment, FieldCacheImpl has somewhat disconcerting values when sorting on a field
for which multiple values exist for one document. For example, imagine a field "fruit" which
is added to a document multiple times, with the values as follows:
> doc 1: {"apple"}
> doc 2: {"banana"}
> doc 3: {"apple", "banana"}
> doc 4: {"apple", "zebra"}
> if one sorts on the field "fruit", the loop in FieldCacheImpl.stringsIndexCache.createValue()
(and similarly for the other methods in the various FieldCacheImpl caches) does the following:
>           while ( {
>             retArray[termDocs.doc()] = t;
>           }
> which means that we look over the terms in their natural order and, on each one, overwrite
retArray[doc] with the value for each document with that term. Effectively, this overwriting
means that a string sort in this circumstance will sort by the LAST term lexicographically,
so the docs above will effecitvely be sorted as if they had the single values ("apple", "banana",
"banana", "zebra") which is nonintuitive. To change this to sort on the first time in the
TermEnum seems relatively trivial and low-overhead; while it's not perfect (it's not local-aware,
for example) the behaviour seems much more sensible to me. Interested to see what people think.
> Patch to follow.

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