[LUCENE-6901] Optimize 1D dimensional value indexing

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 6.0
• Lucene Fields: New
• Assignee: Michael McCandless
• Reporter: Michael McCandless
• Created: 19/Nov/15 15:50
• Updated: 09/May/16 18:40
• Resolved: 24/Nov/15 16:06

Description

Dimensional values give a smaller index, and faster search times, for indexing ordered byte[] values across one or more dimensions, vs our existing approaches, but the indexing time is substantially slower.

Since the 1D case is so important/common (numeric fields, range query) I think it's worth optimizing its indexing time. It should also be possible to optimize the N > 1 dimensions case too, but it's more complex ... we can postpone that.

So for the 1D case, I changed the merge method to do a merge sort (like postings) of the already sorted segments dimensional values, instead of simply re-indexing all values from the incoming segments, and this was a big speedup.

I also changed from InPlaceMergeSorter to IntroSorter (this is what postings use, and it's faster but still safe) and this was another good speedup, which should also help the > 1D cases.

Finally, I added a BKDReader.verify method (currently it's dark: NOT called) that walks the index and then check that every value in each leaf block does in fact fall within what the index expected/claimed. This is useful for finding bugs! Maybe we can cleanly fold it into CheckIndex somehow later.