On 6-Nov-07, at 3:02 PM, Paul Elschot wrote:
> On Tuesday 06 November 2007 23:14:01 Mike Klaas wrote:
>> Wait--shouldn't the outer-most BooleanQuery provide
most of this
>> speedup already (since it should be skipTo'ing
between the nested
>> BooleanQueries and the outermost). Is it the
indirection and sub-
>> query management that is causing the performance
difference, or
>> differences in skiptTo behaviour?
>
> The usual Lucene answer to performance questions: it
depends.
>
> After every hit, next() needs to be called on a
subquery before
> skipTo() can be used to find the next hit. It is
currently not
> defined which
> subquery will be used for this first next().
>
> The structure of the scorers normally follows the
structure of
> the BooleanQueries, so the indirection over the deep
subquery
> scores could well be relevant to performance, too.
>
> Which of these factors actually dominates performance
is hard
> to predict in advance. The point of skipTo() is that is
tries to avoid
> disk I/O as much as possible for the first time that
the query is
> executed. Later executions are much more likely to hit
the OS cache,
> and then the indirections will be more relevant to
performance.
>
> I'd like to have a good way to do a performance test on
a first
> query execution, in the sense that it does not hit the
OS cache
> for its skipTo() executions, but I have not found a
good way yet.
Interesting--thanks for the thoughtful answer.
-Mike
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