Clarify ReplicaReadMixed also uses learner replicas

[King-Dylan]

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• [x] I've signed the Contributor License Agreement, which is required for the repository owners to accept my contribution.

What is changed, added or deleted? (Required)

Close issue 22169 Currently, the official docs say these modes will try leader and follower replicas only.

However, from the KV client implementation:

• leader-and-follower, closest-replicas, and closest-adaptive are classified as ReplicaReadMixed
• ReplicaReadMixed will also attempt to read from learner replicas

This causes a behavior–documentation mismatch. Users who rely on the docs may assume that learners are never used under these modes, which can be surprising, especially when they deploy read-only / learner-only nodes.

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• [ ] v5.4 (TiDB 5.4 versions)

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[gemini-code-assist[bot]]

Summary of Changes

Hello @King-Dylan, I'm Gemini Code Assist[^1]! 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 enhances the accuracy of the TiDB documentation by clarifying the behavior of the tidb_replica_read system variable. Specifically, it updates the descriptions for the leader-and-follower and closest-replicas read modes to explicitly mention that learner replicas are also utilized for read operations, providing a more complete and precise understanding for users configuring replica read policies.

Highlights

• Documentation Clarity for Replica Read Modes: The documentation for the tidb_replica_read system variable has been updated to explicitly state that the leader-and-follower mode also considers learner replicas for read operations, ensuring a more accurate description of the system's behavior.
• Inclusion of Learner Replicas in Closest Replicas Mode: The description for the tidb_replica_read setting when set to closest-replicas has been clarified to include learner replicas as potential candidates for read operations within the same availability zone.

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[LGTM Timeline notifier]

Timeline:

• 2025-12-15 09:09:53.209560533 +0000 UTC m=+1464138.023338095: :ballot_box_with_check: agreed by hfxsd.

[hfxsd]

/approve

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[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-7.1: #22182. But this PR has conflicts, please resolve them!

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #22183.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #22184.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #22185.