Adrien Grand created LUCENE-6585:
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Summary: Make ConjunctionDISI flatten sub ConjunctionDISI instances
Key: LUCENE-6585
URL: https://issues.apache.org/jira/browse/LUCENE-6585
Project: Lucene - Core
Issue Type: Improvement
Reporter: Adrien Grand
Priority: Minor
Today ConjunctionDISI wraps some sub (two-phase) iterators. I would like to
improve it by flattening sub iterators when they implement ConjunctionDISI. In
practice, this would make "+A +(+B +C)" be executed more like "+A +B +C" (only
in terms of matching, scoring would not change).
My motivation for this is that if we don't flatten and are unlucky, we can
sometimes hit some worst cases. For instance consider the 3 following postings
lists (sorted by increasing cost):
A: 1, 1001, 2001, 3001, ...
C: 0, 2, 4, 6, 8, 10, 12, 14, ...
B: 1, 3, 5, 7, 9, 11, 13, 15, ...
If we run "+A +B +C", then everything works fine, we use A as a lead, and
advance B 1000 by 1000 to find the next match (if any).
However if we run "+A +(+B +C)", then we would iterate B and C 2 by 2 over the
entire doc ID space when trying to find the first match which occurs on or
after A:1.
This is an extreme example which is unlikely to happen in practice, but
flattening would also help a bit on some more common cases. For instance
imagine that A, B and C have respective costs of 100, 10 and 1000. If you
search for "+A +(+B +C)", then we will use the most costly iterator (C) to
confirm matches of B (the least costly iterator, used as a lead) while it would
have been more efficient to confirm matches of B with A first, since A is less
costly than C.
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