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## What have you changed? (mandatory)

Please explain **IN DETAIL** what the changes are in this PR and why they are needed:
- Summarize your change (mandatory)
- How does this PR work? Need a brief introduction for the changed logic (optional)
- Describe clearly one logical change and avoid lazy messages (optional)
- Describe any limitations of the current code (optional)

Please **NOTE** that:
- Do not assume the code is self-evident
- Do not assume reviewers understand the original issue

## What are the type of the changes (mandatory)?

The currently defined types are listed below, please pick one of the types for this PR by removing the others:
- New feature (non-breaking change which adds functionality)
- Improvement (non-breaking change which is an improvement to an existing feature)
- Bug fix (non-breaking change which fixes an issue)
- Breaking change (fix or feature that would cause existing functionality to not work as expected)

## How has this PR been tested (mandatory)?

Please describe the tests that you ran to verify your changes. Have you finished unit tests, integration tests, or manual tests?

## Does this PR affect documentation (docs/docs-cn) update? (optional)

If there is document change, please file a PR in ([docs](https://github.com/pingcap/docs) and [docs-cn](https://github.com/pingcap/docs-cn)) and add the PR number here.

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