DNM: 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: support explain verbose mode #23926 verbose explain
2. planner: let CopTiFlashConcurrencyFactor inflence the cost of whole plan #24157 CopTiFlashConcurrencyFactor
3. [reverted]planner: support set tidb_allow_mpp to 2 or ENFORCE to enforce use mpp mode. #24516 support allow_mpp=2
4. 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
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 o fjoin

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-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

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

Details

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[ti-chi-bot]

@LittleFall: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

AFTER it has been approved by collaborators, please ping the release team in a comment to request a cherry pick review.

Details

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/test-infra repository.

[LittleFall]

committed by #26382