expression: fix incorrect copy of flag in truncate function

[tuziemon]

What problem does this PR solve?

Issue Number: close #53546

Problem Summary:
When truncate([const], NULL) is NULL, truncate([const], NULL) is not NULL evaluates to 1 unexpectedly.
In master, all the flags of arg0 are copied to a truncate function.
If arg0 is a constant, NotNullFlag is copied, leading to an unexpected constant fold.

What changed and how does it work?

Only UnsignedFlag is copied, which is similar behavior to the round function.
round([const], NULL) is also NULL, but round([const], NULL) is not NULL returns 0 as expected, so it is used as the reference.

The root cause of this problem refers to #53546 (comment)

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

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[hawkingrei]

/ok-to-test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.4356%. Comparing base (1ee9efa) to head (b0c3ab1).
Report is 750 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #55284        +/-   ##
================================================
+ Coverage   72.9271%   79.4356%   +6.5084%     
================================================
  Files          1570       1730       +160     
  Lines        439488     534740     +95252     
================================================
+ Hits         320506     424774    +104268     
+ Misses        99314      89512      -9802     
- Partials      19668      20454       +786     

Flag	Coverage Δ
integration	52.9330% <100.0000%> (?)
unit	77.2592% <100.0000%> (+5.2912%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	56.1916% <ø> (+3.2349%)	⬆️
parser	∅ <ø> (∅)
br	54.2903% <ø> (+8.3982%)	⬆️

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[tuziemon]

@hawkingrei
could you review this pr when you are available?

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wshwsh12, XuHuaiyu

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Needs approval from an approver in each of these files:

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-11-15 09:33:53.117906202 +0000 UTC m=+607995.308775199: ☑️ agreed by XuHuaiyu.
• 2024-11-15 09:38:16.219106931 +0000 UTC m=+608258.409975912: ☑️ agreed by wshwsh12.

[tuziemon]

/retest

[tuziemon]

/retest

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #57485.