planner: cherry-pick enforce-mpp to v5.0

[LittleFall]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

5.0 needs enforce-mpp.

What is changed and how it works?

cherry-pick these prs to 5.0:

1. planner: let CopTiFlashConcurrencyFactor inflence the cost of whole plan #24157 CopTiFlashConcurrencyFactor
2. [reverted]planner: support set tidb_allow_mpp to 2 or ENFORCE to enforce use mpp mode. #24516 support allow_mpp=2
3. planner: Revert tidb_allow_mpp modification for downgrade compatibility and add warnings for enforce mpp. #25302 revert 24516 and support tidb_enforce_mpp and add warning
4. config: add config for tidb_enforce_mpp and remove the global scope of it. #25369 add config
5. planner: enforce projection push down #25450 enforce projection push down
6. planner: Log warnings when agg function can not be pushdown in explain statement #25553 fix warning
7. planner: add warnings about joins when tidb_enforce_mpp is enabled. #26246 add warning of join

Check List

Tests

• Unit test
• Integration test

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

• Support set tidb_enforce_mpp=1 to enforce use mpp mode.

[ti-srebot]

No release note, Please follow https://github.com/pingcap/community/blob/master/contributors/release-note-checker.md

[LittleFall]

/run-check_release_note

[LittleFall]

/cc @fzhedu @hanfei1991

[LittleFall]

/merge

[ti-chi-bot]

@LittleFall: /merge is only allowed for the committers, you can assign this pull request to the committer in list by filling /assign @committer in the comment to help merge this pull request.

In response to this:

/merge

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[LittleFall]

/merge

[LittleFall]

/merge

[ti-chi-bot]

@LittleFall: /merge is only allowed for the committers, you can assign this pull request to the committer in list by filling /assign @committer in the comment to help merge this pull request.

In response to this:

/merge

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 ti-community-infra/tichi repository.

[LittleFall]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7b44ab7

[hanfei1991]

/merge

[ti-chi-bot]

@LittleFall: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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 ti-community-infra/tichi repository.