Explore other in-memory postinglist formats for realtime search
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Key: LUCENE-2346
URL: https://issues.apache.org/jira/browse/LUCENE-2346
Project: Lucene - Java
Issue Type: Improvement
Components: Index
Reporter: Michael Busch
Assignee: Michael Busch
Priority: Minor
Fix For: 3.1
The current in-memory posting list format might not be optimal for searching.
VInt decoding performance and the lack of skip lists would arguably be the
biggest bottlenecks.
For LUCENE-2312 we should investigate other formats.
Some ideas:
- PFOR or packed ints for posting slices?
- Maybe even int[] slices instead of byte slices? This would be great for
search performance, but the additional memory overhead might not be acceptable.
- For realtime search it's usually desirable to evaluate the most recent
documents first. So using backward pointers instead of forward pointers and
having the postinglist pointer point to the most recent docID in a list is
something to consider.
- Skipping: if we use fixed-length postings ([packed] ints) we can do binary
search within a slice. We can also locate a pointer then without scanning and
thus skip entire slices quickly. Is that sufficient or would we need more
skipping layers, so that it's possible to skip directly to particular slices?
It would be awesome to find a format that doesn't slow down "normal" indexing,
but is very efficient for in-memory searches. If we can't find such a fits-all
format, we should have a separate indexing chain for real-time indexing.
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