Fix outdated statement about TiDB's default transaction mode in optimistic-transaction.md (#21921)

[ti-chi-bot]

This is an automated cherry-pick of #21921

Issue

The documentation in optimistic-transaction.md contained an outdated statement claiming "TiDB uses optimistic concurrency control by default", which has been incorrect since TiDB v3.0.8.

As documented in the v3.0.8 release notes, the default value of the tidb_txn_mode variable was changed from "" (optimistic) to "pessimistic" for newly created clusters starting from v3.0.8 (#14171).

Changes

Updated line 72 in optimistic-transaction.md to accurately reflect the current default behavior:

Before:
> TiDB uses optimistic concurrency control by default, whereas MySQL applies pessimistic concurrency control.

After:
> Starting from v3.0.8, TiDB uses the pessimistic transaction mode by default, the same as MySQL. This means that TiDB and MySQL add locks during the execution of write-type SQL statements, and its Repeatable Read isolation level allows for current reads, so commits generally do not encounter exceptions.

This change adds the temporal qualifier "Starting from v3.0.8" and updates the comparison with MySQL to indicate both databases now use pessimistic concurrency control by default.

Consistency

This update makes the documentation internally consistent:

• The Note at line 15 of the same file already correctly states: "Starting from v3.0.8, TiDB uses the pessimistic transaction mode by default"
• The pessimistic-transaction.md file documents the same default change
• The troubleshoot-write-conflicts.md file also correctly uses the historical context: "Before TiDB v3.0.8..."

Fixes #21902

Original prompt

---

This section details on the original issue you should resolve

<issue_title>"TiDB uses optimistic concurrency control by default" does not look like valid since TiDB 3.0.8 tidb_txn_mode changed default value to pessimistic</issue_title>
<issue_description>File: /master/optimistic-transaction.md

Current description

says

TiDB uses optimistic concurrency control by default

Expected one

Since TiDB 3.0.8, TiDB uses pessimistic concurrency control by default. The document linked above should follow the default behavior.

https://docs.pingcap.com/tidb/stable/release-3.0.8/

Update the default value of the tidb_txn_mode variable from "" to "pessimistic" when a new cluster is created #14171</issue_description>

<agent_instructions>Update the optimistic-transaction.md file accordingly.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@yahonda "TiDB uses optimistic concurrency control by default whereas MySQL applies pessimistic concurrency control. " statement was added via https://github.com//pull/2088

Fixes #21902

---

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[gemini-code-assist]

Summary of Changes

Hello @ti-chi-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical documentation inaccuracy in optimistic-transaction.md. It updates a statement regarding TiDB's default transaction mode to reflect the change introduced in v3.0.8, where the default shifted from optimistic to pessimistic concurrency control. This ensures that users receive current and consistent information about TiDB's behavior, improving the overall accuracy and reliability of the documentation.

Highlights

• Documentation Correction: Corrected an outdated statement in optimistic-transaction.md that incorrectly claimed TiDB uses optimistic concurrency control by default.
• Default Transaction Mode: Updated the documentation to accurately state that TiDB has used pessimistic transaction mode by default since v3.0.8, aligning with MySQL's default behavior.
• Documentation Consistency: Ensured consistency across related documentation files regarding TiDB's default transaction mode, referencing existing correct statements in other sections and files.

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[gemini-code-assist]

Code Review

This pull request correctly updates an outdated statement about TiDB's default transaction mode in optimistic-transaction.md. My review focuses on minor grammatical corrections to improve clarity and accuracy, as per the repository's style guide for automated pull requests. I've suggested a small change to the paragraph on line 71 to fix pronoun agreement and tense.

[hfxsd]

/approve

[ti-chi-bot]

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