executor: open childExec during execution for UnionExec (#24899)

[ti-srebot]

cherry-pick #24899 to release-5.1
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# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/25334

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git push git@github.com:ti-srebot/tidb.git pr/25334:release-5.1-56b6797ab1fc

---

What problem does this PR solve?

Issue Number: close #21441

re-revert #21561

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:
Before this commit, Union opens all the child executors during the Open phase.
This increases the overhead of buildRequest and the risk of OOM.

After this commit, the Open of childExec in Union is moved to the Next phase. The childExec be opened simultaneously is limited to tidb_executor_concurrency atmost.

How it Works:

Related changes

N/A

Check List

Tests

• Integration test
• Manual test
  Suppose we have a table t, it contains 100 partitions, each partition contains 10,000,000 tuples.
  The result of select * from t;
  Before:
  
  After:
  The query works fine.

Side effects

• The speed of UnionExec may decrease if the child of Union is larger than tidb_executor_concurrency.

Release note

• Refine the execution mechanism UnionExec to reduce the overhead of UnionExec.

[ti-srebot]

/run-all-tests

[ti-srebot]

@XuHuaiyu you're already a collaborator in bot's repo.

[guo-shaoge]

/cc @ichn-hu

[guo-shaoge]

/cc @wshwsh12

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• ichn-hu
• wshwsh12

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[wshwsh12]

/merge

[ti-chi-bot]

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

Commit hash: 59b8939

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: Your PR was out of date, I have automatically updated it for you.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