FAQ

On Tuesday 07 April 2009 05:04:44 Daniel Noll wrote:
Hi all.

This is something I have been wondering for a while but can't find a
good answer by reading the code myself.

If you have a query like this:

( field:Value1 OR
field:Value2 OR
field:Value3 OR
... )

How many TermEnum / TermDocs scans should this execute?

(a) One per clause, or
(b) One for the entire boolean query?
One per clause.
I wonder because we do use a lot of queries of this nature, and I can't
find any direct evidence that they get logically merged, leading me to
believe that it's one per clause at present (and thus this becomes a
potential optimisation.)
The problem is not only in the scanning of the TermDocs, but also in
the merging by docId (on a heap) that has to take place when more of them
are used at the same time during the query search.

Some optimisations are already in place:
- By allowing docs scored out of order, most top level OR queries
can be merged with a faster algorithm (distributive sort over docId ranges)
using the term frequencies (see BooleanQuery.setAllowDocsOutOfOrder())
- Various Filters that merge into a bitset, using a single TermDocs
and ignoring term frequencies, (see MultiTermQuery.getFilter()).
- The new TrieRangeFilter that premerges ranges at indexing time,
also ignoring term frequencies.

Using the TermDocs one by one has another advantage in that it
reduces disk seek distances in the index. This is noticeable when
disks have heads that take more time to move longer distances.
SSD's don't have moving heads, so they have smaller performance
differences between merging into a bitset, by distributive sort,
and by a heap.

For the time being, Lucene does not have a low level facility for key values
that occur at most once per document field, so for these it normally helps
to use a Filter.

Regards,
Paul Elschot

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postedApr 7, '09 at 3:04a
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