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From "Mark Miller (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1372) Proposal: introduce more sensible sorting when a doc has multiple values for a term
Date Thu, 04 Sep 2008 13:03:44 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1372?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628331#action_12628331
] 

Mark Miller commented on LUCENE-1372:
-------------------------------------

Hey Paul,

I agree that your patch is more intuitive than the current behavior, but I don't know how
intuitive that is - if the sort worked on multiple tokens, you would expect it to sort lexicographically
across each word, and even with your patch it won't, it will just use the first word rather
than the last, right? In other words, I see it as a half fix.

So while its low overhead, I wonder if any overhead is worth not getting the full fix? Currently
the solution has been that you should only be sorting on single token fields - in fact, there
is a check for this (that just isnt very good at checking <g>) that will possibly throw
an exception if you sort on a field with multiple tokens - its just not safe unless that check
is taken out (FieldCacheImpl string sorting).

It appears that to do this right, we need to pay a cost in the general case and sorting across
multiple tokens may not be worth that, as you can get around the limitation by using multiple
fields etc now. Personally though, if a patch were to be accepted, I think it would have to
fully support the correct sorting and disable that check I mentioned (again, i doubt people
want to pay that perf cost though). Finally, even if the committers decide this is a good
way to go, the check needs to come out at a minimum.




> Proposal: introduce more sensible sorting when a doc has multiple values for a term
> -----------------------------------------------------------------------------------
>
>                 Key: LUCENE-1372
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1372
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>    Affects Versions: 2.3.2
>            Reporter: Paul Cowan
>            Priority: Minor
>         Attachments: 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 (termDocs.next()) {
>             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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