[LUCENE-10033] Encode doc values in smaller blocks of values, like postings

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Won't Fix
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Adrien Grand
• Created: 22/Jul/21 17:31
• Updated: 28/Aug/22 16:23
• Resolved: 13/Sep/21 16:50

Description

This is a follow-up to the discussion on this thread: https://lists.apache.org/thread.html/r7b757074d5f02874ce3a295b0007dff486bc10d08fb0b5e5a4ba72c5%40%3Cdev.lucene.apache.org%3E.

Our current approach for doc values uses large blocks of 16k values where values can be decompressed independently, using DirectWriter/DirectReader. This is a bit inefficient in some cases, e.g. a single outlier can grow the number of bits per value for the entire block, we can't easily use run-length compression, etc. Plus, it encourages using a different sub-class for every compression technique, which puts pressure on the JVM.

We'd like to move to an approach that would be more similar to postings with smaller blocks (e.g. 128 values) whose values get all decompressed at once (using SIMD instructions), with skip data within blocks in order to efficiently skip to arbitrary doc IDs (or maybe still use jump tables as today's doc values, and as discussed here for postings: https://lists.apache.org/thread.html/r7c3cb7ab143fd4ecbc05c04064d10ef9fb50c5b4d6479b0f35732677%40%3Cdev.lucene.apache.org%3E).

Issue Links

• is depended upon by: LUCENE-7806 Explore delta of delta encoding
• links to: GitHub Pull Request #227