lance-format/lance

Allow dictionary encoding to support more fixed width types

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#5,004 opened on Oct 20, 2025

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good first issue

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Description

Currently the dictionary encoding in 2.1 only applies to some 128-bit fixed-width types. I think this was a combination of a few things:

  • At the time, we were not bit-packing indices, and so applying dictionary encoding to smaller types was unlikely to be beneficial
  • Estimating cardinality on all columns was expensive
  • TPC-H data has a lot of 128-bit decimals that benefit from dictionary encoding so there was an outsized (and unrealistic) benchmarking gain to focusing on 128-bit types.

However, we should at least be considering dictionary encoding for 64-bit types and maybe even 32-bit types.

Contributor guide

Research direction
Examine the current dictionary encoding implementation, focusing on how fixed width types are selected for encoding. Extend the selection to include 64 bit and 32 bit fixed width types, considering cardinality estimation and performance implications.
Tech stack
rust
Domain
database
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Rustencoding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45