Fix outdated statement about TiDB's default transaction mode in optimistic-transaction.md

[Copilot]

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The documentation in optimistic-transaction.md contained an outdated statement claiming "TiDB uses optimistic concurrency control by default", which has been incorrect since TiDB v3.0.8.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

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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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[ti-chi-bot]

Hi @Copilot. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[pingcap-cla-assistant]

All committers have signed the CLA.

[lilin90]

/ok-to-test

[lilin90]

/lgtm

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-10-16 08:19:55.333007449 +0000 UTC m=+341501.410260019: ☑️ agreed by lilin90.
• 2025-10-16 08:48:23.223500437 +0000 UTC m=+343209.300752987: ☑️ agreed by hfxsd.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #21924.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #21925.