session: add index on 'user' field for mysql tables | tidb-test=pr/2430 (#57470)

[ti-chi-bot]

This is an automated cherry-pick of #57470

What problem does this PR solve?

Issue Number: ref #55563

Problem Summary:

What changed and how does it work?

Another solution for the same purpose, see #57320
If this one is chosen, that one could be closed.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

create new cluster and upgrade old cluster, show create table to see the result.

• No need to test

  • I checked and no code files have been changed.

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

@tiancaiamao This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign benmeadowcroft, leavrth for approval, ensuring that each of them provides their approval before proceeding. For more information see the Code Review Process.

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Details

Needs approval from an approver in each of these files:

• OWNERS
• br/OWNERS
• pkg/session/OWNERS

Approvers can indicate their approval by writing /approve in a comment
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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/unit-test	ab98500	link	true	/test unit-test
idc-jenkins-ci-tidb/build	ab98500	link	true	/test build
idc-jenkins-ci-tidb/check_dev_2	ab98500	link	true	/test check-dev2
idc-jenkins-ci-tidb/check_dev	ab98500	link	true	/test check-dev
pull-br-integration-test	ab98500	link	true	/test pull-br-integration-test
idc-jenkins-ci-tidb/mysql-test	ab98500	link	true	/test mysql-test

Full PR test history. Your PR dashboard.

Details

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[tiancaiamao]

new feature, don't cherry-pick to 8.5