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From "Mark Miller (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1821) Weight.scorer() not passed doc offset for "sub reader"
Date Fri, 21 Aug 2009 01:04:14 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1821?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12745756#action_12745756
] 

Mark Miller commented on LUCENE-1821:
-------------------------------------

{quote}
howevever, this method is actually also a bit broken now with per segment searching
what if reader is a MultiReader
{quote}

Right - there are many places where this could be the case - your still free to use multi-readers,
though we encourage you to switch. We provide a cool cache sanity checker to help you find
these cases, and evaluate whether or not you can make the switch.

If you just pass 0, many times it will be wrong. Why shouldn't this have access to a doc id
cache as well? We always ask for everything in the context of the Reader given. I think thats
the issue. Lucene just never officially supported this use case - we can't with MultiSearcher,
Searchable, Remote - the API doesn't work with the idea that you can count on all the doc
ids from a Reader. You were taking advantage of the implementation and your limited use of
the full API - but its never been part of the API IMHO.

Perhaps we could one day change things - RMI hasn't really worked out in comparison to other
methods large scale (supposedly very chatty - though I have been told very large installations
have been built with it ) - we have already factored it into contrib. But this still doesn't
fit the current model/API, and if we address it, it will take longer than 2.9 to do right
IMO.

> Weight.scorer() not passed doc offset for "sub reader"
> ------------------------------------------------------
>
>                 Key: LUCENE-1821
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1821
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Search
>    Affects Versions: 2.9
>            Reporter: Tim Smith
>             Fix For: 2.9
>
>         Attachments: LUCENE-1821.patch
>
>
> Now that searching is done on a per segment basis, there is no way for a Scorer to know
the "actual" doc id for the document's it matches (only the relative doc offset into the segment)
> If using caches in your scorer that are based on the "entire" index (all segments), there
is now no way to index into them properly from inside a Scorer because the scorer is not passed
the needed offset to calculate the "real" docid
> suggest having Weight.scorer() method also take a integer for the doc offset
> Abstract Weight class should have a constructor that takes this offset as well as a method
to get the offset
> All Weights that have "sub" weights must pass this offset down to created "sub" weights
> Details on workaround:
> In order to work around this, you must do the following:
> * Subclass IndexSearcher
> * Add "int getIndexReaderBase(IndexReader)" method to your subclass
> * during Weight creation, the Weight must hold onto a reference to the passed in Searcher
(casted to your sub class)
> * during Scorer creation, the Scorer must be passed the result of YourSearcher.getIndexReaderBase(reader)
> * Scorer can now rebase any collected docids using this offset
> Example implementation of getIndexReaderBase():
> {code}
> // NOTE: more efficient implementation can be done if you cache the result if gatherSubReaders
in your constructor
> public int getIndexReaderBase(IndexReader reader) {
>   if (reader == getReader()) {
>     return 0;
>   } else {
>     List readers = new ArrayList();
>     gatherSubReaders(readers);
>     Iterator iter = readers.iterator();
>     int maxDoc = 0;
>     while (iter.hasNext()) {
>       IndexReader r = (IndexReader)iter.next();
>       if (r == reader) {
>         return maxDoc;
>       } 
>       maxDoc += r.maxDoc();
>     } 
>   }
>   return -1; // reader not in searcher
> }
> {code}
> Notes:
> * This workaround makes it so you cannot serialize your custom Weight implementation

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