pingcap/tidb

Support to use the outer side of the (Anti) Semi Join to build the hash table

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#32,335 opened on Feb 14, 2022

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help wantedsig/plannertype/enhancement

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Description

Enhancement

Currently, when TiDB meets a(an) (Anti) Semi join, it would always choose the inner side to build the hash table. When the inner side is extremely large compared with the outer side, our current strategy would cost a lot on building the hash table by the inner side.

So we should support constructing the hash table by the outer side to speed up the execution when the data distribution is like the above case.

It's something the same with supporting constructing hash table by the outer side of the outer join, which is implemented in TiDB.

Contributor guide

Research direction
Examine the existing implementation for building hash table on outer side of outer joins in TiDB. Then apply a similar logic for semi/anti semi joins, focusing on the case when the inner side is large relative to the outer side. Test with varying data distributions.
Tech stack
go
Domain
backenddatabase
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
Godatabase internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60