Fast bulk explains?

[Gregg Donovan <gr...@gmail.com>]

We're attempting to get explains for the top 1000 documents we fetch from
Lucene and running into performance problems. Is there a good way to speed
these up?

In our profiling, we see the advance() methods in each of the subqueries
dominating the CPU time. The explain API takes 1 document at a time and
each subquery scorer does an advance(), so the time seems to be a function
of docs * queryTerms.

I can't see an easy way to speed this up. Maybe making a temporary
in-memory index that contained the 1000 docs to minimize the cost of
advance()? Maybe an AtomicReader that filters anything but the docs we care
about?

Thanks.

--Gregg