pingcap/tidb

Partial2Mode is not necessary in hash aggregation

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#15,271 opened on Mar 10, 2020

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help wantedsig/executiontype/enhancementtype/performance

Description

Feature Request

Is your feature request related to a problem? Please describe:

hash aggregation with Coprocessor fetches partial results from storage and runs

  1. Partial2Mode, partial results --> partial results
  2. FinalMode, partial results --> final results

It seems that Partial2Mode is a redundant run, we can remove it for better code and performance.

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

Contributor guide

Tech stack
go
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.
half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Go programmingTiDB aggregation pipeline
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Investigate TiDB's coprocessor aggregation code, specifically how Partial2Mode and FinalMode are used. Look at the hash aggregation logic to confirm that Partial2Mode is redundant and can be removed without affecting correctness. The relevant files are likely in the coprocessor package. Check if there are any tests that cover this behavior.