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From "Eks Dev (JIRA)" <>
Subject [jira] Commented: (LUCENE-1518) Merge Query and Filter classes
Date Thu, 30 Apr 2009 10:30:30 GMT


Eks Dev commented on LUCENE-1518:

imo, it is really not all that important to make Filter and Query the same (that is just one
alternative to achieve goal). 

Basic problem we try  to solve is adding Filter directly to BoolenQuery, and making optimizations
after that easier. Wrapping with CSQ is just adding anothe layer between Lucene search machinery
and Filter, making these optimizations harder.

On the other hand, I must accept, conceptually FIter and Query are "the same", supporting
together following options:
1. Pure boolean model: You do not care about scores (today we can do it only wia CSQ, as Filter
does not enter BoolenQuery)
2. Mixed boolean and ranked: you have to define Filter contribution to the documents (CSQ)
3. Pure ranked: No filters, all gets scored (the same as 2.)

Ideally, as a user, I define only Query (Filter based or not) and for each clause in my Query
Query.setScored(true/false) or useConstantScore(double score); 

also I should be able to say, "Dear Lucene please materialize this "Query_Filter" for me as
I would like to have it cached and please store only DocIds (Filter today).  Maybe open possibility
to open possibility to cache scores of the documents as well. 

one thing is concept  and another is optimization. From optimization point of view, we have
couple of decisions to make:

- DocID Set supports random access, yes or no (my "Materialized Query")
- Decide if clause should / should not be scored/ or should be constant

So, for each "Query" we need to decide/support:

- scoring{yes, no, constant} and 
- opening option to "materialize Query" (that is how we today create Filters today)
- these Materialized Queries (aka Filter) should be able to tell us if they support random
access, if they cache only doc id's or scores as well

nothing usefull in this email, just  thinking aloud, sometimes helps :)

> Merge Query and Filter classes
> ------------------------------
>                 Key: LUCENE-1518
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>    Affects Versions: 2.4
>            Reporter: Uwe Schindler
>             Fix For: 2.9
>         Attachments: LUCENE-1518.patch
> This issue presents a patch, that merges Queries and Filters in a way, that the new Filter
class extends Query. This would make it possible, to use every filter as a query.
> The new abstract filter class would contain all methods of ConstantScoreQuery, deprecate
ConstantScoreQuery. If somebody implements the Filter's getDocIdSet()/bits() methods he has
nothing more to do, he could just use the filter as a normal query.
> I do not want to completely convert Filters to ConstantScoreQueries. The idea is to combine
Queries and Filters in such a way, that every Filter can automatically be used at all places
where a Query can be used (e.g. also alone a search query without any other constraint). For
that, the abstract Query methods must be implemented and return a "default" weight for Filters
which is the current ConstantScore Logic. If the filter is used as a real filter (where the
API wants a Filter), the getDocIdSet part could be directly used, the weight is useless (as
it is currently, too). The constant score default implementation is only used when the Filter
is used as a Query (e.g. as direct parameter to For the special case of
BooleanQueries combining Filters and Queries the idea is, to optimize the BooleanQuery logic
in such a way, that it detects if a BooleanClause is a Filter (using instanceof) and then
directly uses the Filter API and not take the burden of the ConstantScoreQuery (see LUCENE-1345).
> Here some ideas how to implement with Query and Filter:
> - User runs using a Filter as the only parameter. As every Filter is
also a ConstantScoreQuery, the query can be executed and returns score 1.0 for all matching
> - User runs using a Query as the only parameter: No change, all is
the same as before
> - User runs using a BooleanQuery as parameter: If the BooleanQuery
does not contain a Query that is subclass of Filter (the new Filter) everything as usual.
If the BooleanQuery only contains exactly one Filter and nothing else the Filter is used as
a constant score query. If BooleanQuery contains clauses with Queries and Filters the new
algorithm could be used: The queries are executed and the results filtered with the filters.
> For the user this has the main advantage: That he can construct his query using a simplified
API without thinking about Filters oder Queries, you can just combine clauses together. The
scorer/weight logic then identifies the cases to use the filter or the query weight API. Just
like the query optimizer of a RDB.

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