pingcap/tidb

Provide a session variable to query the last transaction scope

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#20,775 opened on Nov 2, 2020

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good first issuesig/transactiontype/enhancement

Description

Development Task

To figure out which transaction scope the last transaction uses, we need a read-only session variable to query it. This is a part of the global/local transaction roadmap #20448.

Implementations

It's similar to other session variables like last_insert_id and last_plan_from_cache. You can refer to the implementation of these variables.

Contributor guide

Tech stack
gosql
Domain
backenddatabase
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.
clear
Prerequisites
Go programming knowledgeUnderstanding of TiDB session variablesFamiliarity with TiDB codebase structure
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
First, study existing session variables like `last insert id` and `last plan from cache` in the TiDB codebase, particularly in the `sessionctx/variable` directory. Identify where these variables are defined and how they are set and queried. Then, implement a new read only variable for the last transaction scope, following the same pattern. Pay attention to the transaction context and ensure the variable is updated appropriately at transaction boundaries. Refer to the roadmap issue #20448 for overarching context.