planner: fix stream agg pushed down to mpp plan unexpectedly

[time-and-fate]

Issue Number: close #32632

Problem Summary:

In (*PhysicalStreamAgg).attach2Task(), we didn't consider that it's possible that the task could be an mpp task even though we don't want an mpp task.
Most times this would not happen, but it's possible when tidb_enforce_mpp is turned on. (reference: #25450: https://github.com/pingcap/tidb/pull/25450/files#diff-d3d0144d9fe67ce1ca77fb02c6ebb7895cade0809e47b21b926bfa29feb0efb2)

When this happened, the mpp task is still considered as a root task, then the stream agg will be put directly on the plan tree of the task. This will essentially push the stream agg to the mpp task without remaining a phase 2 agg in the tidb side.

What is changed and how it works?

Add a check for mpp task in (*PhysicalStreamAgg).attach2Task(), finish the mpp part of the plan tree and remain the stream agg in the tidb side, and avoid unexpectedly pushing stream agg down to a mpp task.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

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

None

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[time-and-fate]

/merge

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

Details

Commit hash: d9a3f35

[time-and-fate]

/run-unit-test

[LittleFall]

LGTM

[time-and-fate]

/run-unit-test

[time-and-fate]

/merge

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

Details

Commit hash: af93d6e

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Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/e0e1aa5c796cd899a57899e27d26189179507182