[LUCENE-9536] Optimize OrdinalMap when one segment contains all distinct values?

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: 8.8
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Julie Tibshirani
• Created: 21/Sep/20 21:33
• Updated: 28/Aug/22 16:08
• Resolved: 02/Nov/20 15:49

Description

For doc values that are not too high cardinality, it seems common to have some large segments that contain all distinct values (plus many small segments who are missing some values). In this case, we could check if the first segment ords map perfectly to global ords and if so store `globalOrdDeltas` and `firstSegments` as `LongValues.ZEROES`. This could save a small amount of space.

I don’t think it would help a huge amount, especially since the optimization might only kick in with small/ medium cardinalities, which don’t create huge `OrdinalMap` instances anyways? But it is simple and seemed worth mentioning.

Issue Links

• links to: GitHub Pull Request #1948
• links to: GitHub Pull Request #2070