pingcap/tidb

Non-transactional DML should not use the default scan concurrency

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#67329 opened on Mar 26, 2026

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good first issuetype/enhancement

Description

Enhancement

According to https://github.com/pingcap/tidb/blob/fd7c13a9340470233bf0c52c60eb8c04095d394c/pkg/distsql/request_builder.go#L94, when max_distsql_concurrency is the default setting, the first SELECT in non-transactional DML may only use max_distsql_concurrency=2, which affects its performance.

Contributor guide

Tech stack
gosql
Domain
databaseperformance
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.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Go programmingTiDB internalsdistsql package
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
50
Research direction
The issue points to `pkg/distsql/request builder.go` line 94 where the default `max distsql concurrency` is used. Investigate why non transactional DML uses this default and how to bypass it to improve performance. Check if a similar pattern exists for transactional queries and whether a different concurrency value should be passed. Review the surrounding code and any related configuration options.