expand streams after agg (#2530)

[ti-srebot]

cherry-pick #2530 to release-5.1
You can switch your code base to this Pull Request by using git-extras:

# In tics repo:
git pr https://github.com/pingcap/tics/pull/2537

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tics.git pr/2537:release-5.1-a3a663351625

---

What problem does this PR solve?

Issue Number: close #2453

Problem Summary: expand streams after aggregation

What is changed and how it works?

Proposal: xxx

What's Changed:

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Manual test (add detailed scripts or steps below)
  the second agg is parallel, but before this PR, it is non-parallel agg.

["executeQuery: Query pipeline:\nExpression\n Expression\n  ParallelAggregating\n   Expression × 24\n    Expression\n     Expression\n      Expression\n       Expression\n        Expression\n         SimpleInputStream\n          SharedQuery\n           Expression\n            Expression\n             ParallelAggregating\n              Expression × 24\n               Squashing\n                TiRemoteBlockInputStream(ExchangeReceiver)\n"] [thread_id=485]

Side effects

Release note

• expand streams after aggregation

[fzhedu]

/LGTM

[fzhedu]

/run-all-tests

[windtalker]

/merge

[ti-chi-bot]

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[ti-chi-bot]

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

Commit hash: 4fcab35