*: add tiflash_hashagg_preaggregation_mode sys var

[guo-shaoge]

What problem does this PR solve?

Issue Number: close #54867

Problem Summary:

What changed and how does it work?

Check List

Tests

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

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[tiprow]

Hi @guo-shaoge. Thanks for your PR.

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[codecov]

Codecov Report

Attention: Patch coverage is 79.24528% with 11 lines in your changes missing coverage. Please review.

Project coverage is 75.0533%. Comparing base (a541a4f) to head (74913f9).
Report is 3 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #54186        +/-   ##
================================================
+ Coverage   72.8356%   75.0533%   +2.2177%     
================================================
  Files          1570       1590        +20     
  Lines        439557     469522     +29965     
================================================
+ Hits         320154     352392     +32238     
+ Misses        99659      95907      -3752     
- Partials      19744      21223      +1479     

Flag	Coverage Δ
integration	76.5438% <79.1666%> (?)

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Components	Coverage Δ
dumpling	∅ <ø> (∅)
parser	∅ <ø> (∅)
br	53.2528% <ø> (+7.3907%)	⬆️

[ti-chi-bot]

PR needs rebase.

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[windtalker]

For some agg, it may generate 3 stage aggregation, will the first 2 stage agg be treated as partial agg?

[guo-shaoge]

I checked some 3-stage agg plans and found most 2nd agg has no group by column. So I think the 2nd agg no need to use auto pass through

[winoros]

planner part LGTM

[easonn7]

/approve

[windtalker]

lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: easonn7, windtalker, winoros

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Needs approval from an approver in each of these files:

• OWNERS [windtalker,winoros]
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• pkg/sessionctx/variable/OWNERS [easonn7]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-08-07 07:06:20.334850889 +0000 UTC m=+422110.201949978: ☑️ agreed by winoros.
• 2024-08-08 05:15:17.31290261 +0000 UTC m=+501847.180001698: ☑️ agreed by windtalker.