pingcap/tidb

Pruning the plan branches with dual tables.

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#25,234 opened on Jun 8, 2021

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help wantedsig/plannertype/feature-request

Description

Feature Request

In optimizer, it will replace a forever-empty table (like table scan with a constant false) with a "dual" table. This opt can save the rpc calls to TiKV or TiFlash engine. But this opt still miss some cases, there are two opts that're worthy to do:

  • inner/semi join with dual table. In this case, we can replace inner join with dual table 'cause the result of join is empty too. Moreover, it can save the i/o scan greatly.
  • union all/outer join with dual. In the mpp settings, an operator involved with dual table will block pushing down. Some operators that can run on TiFlash would have to run on TiDB. We have optimize the union all cases, but more need to be done.

Besides, TiFlash should also need to support dual in order to solve corner cases, such as outer join dual tables.

Contributor guide

Tech stack
gosql
Domain
databasebackend
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.
1-2 days
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.
mostly clear
Prerequisites
Go programmingTiDB optimizer basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the current optimizer logic for replacing empty tables with dual in the plan tree. Specifically, examine how joins with dual are currently handled and identify where the missing cases (inner/semi join and union all/outer join) need to be implemented. Look at the planner code in the 'planner/core' directory and consider how TiFlash's support for dual tables could be added.