pingcap/tidb

Loose index scan can be used to speed up DISTINCT upon JOIN

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#18,693 opened on Jul 20, 2020

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challenge-programhelp wantedsig/executionsig/planner

Description

Description

In issue https://github.com/pingcap/tidb/issues/14460. It proposed a scan method called loose index scan. This method is to scan the index and skip the same value.

In this issue, we describe a scene that is not mentioned in that issue.

Consider query select distinct t2.a from t1, t2 where t1.a=t2.a.

Since we need unique t2.a. So we also don't need t1.a return the same value twice, which means that we can apply a loose index scan on the t1 side.

Score

6600

Mentor

@winoros (Slack ID: Cui Yiding)

#sig-planner

Recommend Skills

  • Golang and Rust

Contributor guide

Tech stack
gorust
Domain
database
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Go programmingDatabase indexingSQL JOINsTiDB internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Start by examining the referenced issue #14460 which introduced the loose index scan concept. Then study the TiDB source code in the planner and executor packages to understand current DISTINCT and JOIN processing. Review the comments on this issue for any additional guidance from mentor @winoros. The goal is to implement a new scan method that skips duplicate values on the t1 side of the join.