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From "Tim Smith (JIRA)" <>
Subject [jira] Commented: (LUCENE-1821) Weight.scorer() not passed doc offset for "sub reader"
Date Fri, 21 Aug 2009 13:16:14 GMT


Tim Smith commented on LUCENE-1821:

bq. You never officially had the full index context
Officially, i didn't "not" have the full index context either (it was undefined at best, but
was clear from both lucene code and my use of the API that i did have the full index context)

Whenever i do a search, i always explicitly know what context i'm searching in (its always
an IndexSearcher context)
further, whenever i pass an IndexReader to any method (to create a cache/etc), i explicitly
know what context i'm dealing with in order to know what the docids used mean
as the application developer, i have full control over what i pass into the lucene API and
where, and know the context of passing that in (javadoc should just be fully clear on how
what goes in is used (if not already) (i always have the option to not use a utility class/method
provided by lucene if it does not have the proper context semantics i need (and can write
my own that does)

bq. The current API would not support this without back compat breaks up the wazoo
i kinda see what you mean here, but then how is it ok to pass an IndexReader to this method
by the same right
it seems like it should be ok to pass the IndexSearcher (the direct context for the IndexReader)
for the IndexReader in question to Weight.scorer() if its ok to pass the IndexReader (the
scorer() method's interface was already changed between 2.4 and 2.9 (adding allowDocsInOrder
and topScorer))

bq. You can pick, but we have to be true to the API or change it (not easy with our back compat
be fair, 2.9 has a lot of back compat breaks, both in API and runtime behavior (i had tons
of compile errors when i dropped 2.9 in, as well as some other hacks i had to add in (at least
temporarily) in order to get 2.9 to work due to run time changes (primarily this per segment
search stuff))

I have no problem with back compat breaks in general (only took me about a day to absorb 2.9
initially (still working on fully taking advantage of new features and getting rid of deprecated
class use)) The only requirement i would put on a back compat break is that it have a workaround
to get back the the previous versions behavior (in this case have it possible to remap the
docids to the "IndexSearcher" context inside the scorer)

> Weight.scorer() not passed doc offset for "sub reader"
> ------------------------------------------------------
>                 Key: LUCENE-1821
>                 URL:
>             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);
>       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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