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From "Chris Male (JIRA)" <>
Subject [jira] [Commented] (LUCENE-3041) Support Query Visting / Walking
Date Tue, 26 Apr 2011 11:04:03 GMT


Chris Male commented on LUCENE-3041:

bq. Regarding sharing the map, I think you should use a prototype pattern that creates a new
Processor from an existing one maybe via clone()? In the InvocationDispatcher case we should
maybe use a concurrent hash map and share the map across instances for the same dispatcher

I don't quite follow you.  Currently DispatchingQueryProcessor caches InvocationDispatchers
by concrete impl type.  So we only create a new InvocationDispatcher when we have a new implementation
(which means InvocationDispatchers are shared between segments, searches, everything).  In
that regard DispatchingQueryProcessor#dispatcherByClass should be a ConcurrentHashMap.  But
otherwise, I think we're okay?

bq. The process implementation in DefaultQueryProcessor executes query.rewrite before passing
the query to the dispatcher which is no good since some QueryProcessor impls might not want
to rewrite that query at all. In LUCENE-2868 karl tries to find a way to prevent lucene to
rewrite one and the same FuzzyQuery since he has them in multiple clauses somewhere down the
BQ tree. This is a super expensive operation in his case to rewriting it only once makes sense.
I think this should be left to the actual implementation.

This is super tricky.  The question is how to define a base case in #process(Query).  Lets
assume DefaultQueryProcessor#process(Query) just dispatched immediately.  It might be a receiver
of the same dispatch (perhaps the query is a TermQuery and no #process(TermQuery) is provided,
so #process(Query) is chosen).  It then just dispatches again, receives again.. and we're
in a loop.

Any thoughts on how to avoid that?

> Support Query Visting / Walking
> -------------------------------
>                 Key: LUCENE-3041
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>            Reporter: Chris Male
>            Priority: Minor
>         Attachments: LUCENE-3041.patch
> Out of the discussion in LUCENE-2868, it could be useful to add a generic Query Visitor
/ Walker that could be used for more advanced rewriting, optimizations or anything that requires
state to be stored as each Query is visited.
> We could keep the interface very simple:
> {code}
> public interface QueryVisitor {
>   Query visit(Query query);
> }
> {code}
> and then use a reflection based visitor like Earwin suggested, which would allow implementators
to provide visit methods for just Querys that they are interested in.

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