tiflash: Add warnings about LOCATION LABELS on ALTER TABLE ... SET TIFLASH REPLICA

[JaySon-Huang]

First-time contributors' checklist

• 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)

According to the discussion with @JmPotato , the current implementation of LOCATION LABELS is not correct and may acts does not meet the user want. Fixing the behavior requires manpower, and there is currently no plan or demand from users. So just add warnings about it.

For example, if the user set LOCATION LABELS with the similar string as placement-rule-in-SQL does: https://docs.pingcap.com/zh/tidb/stable/placement-rules-in-sql/#%E6%8C%87%E5%AE%9A%E7%94%9F%E5%AD%98%E5%81%8F%E5%A5%BD

alter table t set tiflash replica 2 LOCATION LABELS "[region, zone, host]"

then the generated placement-rule in PD is

  {
    "group_id": "tiflash",
    ...
    "start_key": "748000000000055fff795f720000000000fa",
    "end_key": "748000000000055fff7a00000000000000f8",
    "role": "learner",
    "count": 2,
    "label_constraints": [
      {
        "key": "engine",
        "op": "in",
        "values": ["tiflash"]
      }
    ],
    "location_labels": [
      "[region, zone, host]"
    ],
  }

but not

  {
    "group_id": "tiflash",
    ...
    "start_key": "748000000000055fff795f720000000000fa",
    "end_key": "748000000000055fff7a00000000000000f8",
    "role": "learner",
    "count": 2,
    "label_constraints": [
      {
        "key": "engine",
        "op": "in",
        "values": ["tiflash"]
      }
    ],
    "location_labels": [
      "region",
      "zone",
      "host"
    ],
  }

Which TiDB version(s) do your changes apply to? (Required)

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s): tiflash: Add warnings about LOCATION LABELS on ALTER TABLE ... SET TIFLASH REPLICA docs#21692

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[JaySon-Huang]

/cc @JmPotato @joey-yez

[ti-chi-bot]

@JmPotato: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-08-29 03:49:06.828816601 +0000 UTC m=+498438.706647545: ☑️ agreed by hfxsd.

[ti-chi-bot]

@joey-yez: adding LGTM is restricted to approvers and reviewers in OWNERS files.

In response to this:

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hfxsd

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [hfxsd]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #20848.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #20849.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #20850.