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From Chris Hostetter <hossman_luc...@fucit.org>
Subject Re: FunctionQuery example request
Date Wed, 15 Mar 2006 20:23:12 GMT

: I have implemented the DistanceComparatorSource
: example from Lucene In Action (my Bible) and it works
: great. We are now in the situation where we have
: nearly a million documents in our index and the
: performance of this implementation has degraded.

: Can someone please spare a couple of minutes to give
: me an example of how I would implement a FunctionQuery
: to score each of the documents matched in my boolean
: query?

First off...

I'm not sure if replacing your DistanceComparatorSource with a
FunctionQuery that conputes the distance will result in a performance
improvment -- either way you'r computing the distance for every match.
Where using a FunctionQuery has it's greatest benefits is when you want
the order of results to be based not just on an equation (like you can
impliment with a SortComparatorSource) but by a score that is heavily
influenced using that equation.

That said, one approach using FunctionQuery that should improve the
performance over a SortComparatorSource that performs the same function,
would be to determine a "bounding box" and wrap your FunctionQuery
in a BooleanQuery with ConstantScoreRangeQueries that would enfoce this
bounding box -- that way your Function will only be asked to perform it's
computation on the items that are "near by" and you wont spend a lot of
time computing the distance on the less important (farther out) results.

For example, if you had a DistanceFunctionQuery, instead of using it like
this (psuedocode)

  Query mainQuery = ...
  BooleanQuery wrapper = new BooleanQuery()
  wrapper.add(mainQuery, MANDATORY)
  wrapper.add(new FunctionQuery(...), OPTIONAL)

...use something like this...

  Query mainQuery = ...
  BooleanQuery wrapper = new BooleanQuery()
  wrapper.add(mainQuery, MANDATORY)
  BooleanQuery sub = new BooleanQuery()
  sub.add(new FunctionQuery(...), OPTIONAL)
  sub.add(new ConstantScoreRangeQuery(lonField, currentLon-buf, currentLon+buf))
  sub.add(new ConstantScoreRangeQuery(latField, currentLat-buf, currentLat+buf))
  wrapper.add(sub, OPTIONAL)

Okay .. all of that said, the best way to understand how to impliment your
own function, is to start by looking at an existing Function that does
some numeric calculation. I would suggest starting with
LinearFloatFunction.

FunctionQueries work by dealing with ValueSources that determine the value
for a document.  LinearFloatFunction is a type of ValueSource that works
by computing a simple calculation on the results of another ValueSource.
ValueSources could be nested pretty much indefinitely, but eventually you
want to deal with acctaul data from the index -- which is where
IntFieldSource or FloatFieldSource come in -- they are very simple
ValueSource implimentations that just returns the value of an indexed
numeric field using the FieldCache.

If you look at the source for LinearFloatFunction. you'll see that it
takes one ValueSource in it's constructor, and then in it's
getValues(IndexReader) method it uses the values from that ValueSource in
a linear equation.

to achieve similar results with a distance equation, you would want your
DistanceFunction to take in two ValueSources (one for the longitude field,
and one for the latitude field) and your range.  then just override the
getValues(IndexReader) function to do the calculation you want.

you would then use it soemthing like this...

   FunctionQuery fq = new FunctionQuery(
          new DistanceFunction(
             new FloatFieldSource("latFieldName"),
             new FloatFieldSource("lonFieldName"),
             currentLat,
             currentLon));



-Hoss


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