[hibernate-dev] Hibernate Search 3.1

[Nick Vincent]

Hi Emmanuel,

On 26/02/2008, Emmanuel Bernard <emmanuel at hibernate.org> wrote:
>
>  On  Feb 26, 2008, at 06:41, Nick Vincent wrote:
>  >
>  >
>  > 2) Explaining results
>  >
>  > This uses the new DOCUMENT_ID projection introduced in 3.0.1  to
>  > explain query results (we need this so the customer can understand
>  > their search results in the backoffice interface).  I added an explain
>  > method to both implementations of FullTextQueryImpl which is only
>  > available by casting (e.g. no interface changes).  I think explain()
>  > is probably a fairly advanced function which it's acceptable to access
>  > by casting.
>
>
> Wouldn't it make sense to expose the explain result (I imagine an
>  Explanation object) as a projected field?

The Lucene javadoc says "Computing an explanation is as expensive as
executing the query over the entire index.".

http://lucene.apache.org/java/2_3_1/api/core/org/apache/lucene/search/Searcher.html#explain(org.apache.lucene.search.Query,%20int)

which is why I didn't consider projecting this.  If that's true the
effort to project an explanation onto the results will increase
exponentially with the number of hits.  For this reason I think the
method of accessing an Explanation that I proposed is reasonable
(although not necessarily right).

>  >
>  > 3) Counting results
>  >
>  > In the current implementation we only want to perform one Lucene query
>  > per search (all projected).  In order to get a resultcount and the
>  > results themselves it is currently necessary to invoke the Lucene
>  > query twice.
>
>
> This is not true.
>
>  query.list(); //triggers a lucene query
>  query.getResultSize(); //does not since list() has already computed it

You are right, and I don't need to make any alterations.  I've worked
out what the problem we encountered was that made me think this was a
problem.  It took a bit of digging around the source to work out what
we'd done wrong, and perhaps it might be useful to include in an FAQ
or the documentation.  If you make the calls in this order:

query.getResultSize();  // Hits retrieved, hitcount cached and returned
query.list(); // Hits retrieved

then the query gets run twice as resultCount is cached in
FullTextQueryImpl but the Hits object is not.

A subtle effect, but when you're using something like JSF you're not
always sure in which order the properties of your underlying beans are
retrieved during the render cycle.  This was the cause of our double
querying behaviour.

Cheers,

Nick