expression: use the correct type when eval decimal and float session var (#51395)

[ti-chi-bot]

This is an automated cherry-pick of #51395

What problem does this PR solve?

Issue Number: close #43527

Problem Summary:

What changed and how does it work?

create table test (a datetime, b bigint, c decimal(10, 2), d float);
insert into test values('2010-10-10 10:10:10', 100, 100.01, 100);
SELECT @total := @total + c FROM (SELECT c FROM test) AS temp, (SELECT @total := 200) AS T1;

In the SQL, @total type is int64, but when we do eval decimal we will directly convert it as decimal then we will get a panic because there is no x for int value.
GetMysqlDecimal:

func (d *Datum) GetMysqlDecimal() *MyDecimal {
	return d.x.(*MyDecimal)
}

ToDecimal:

func (d *Datum) ToDecimal(ctx Context) (*MyDecimal, error) {
	switch d.Kind() {
	case KindMysqlTime:
		return d.GetMysqlTime().ToNumber(), nil
	case KindMysqlDuration:
		return d.GetMysqlDuration().ToNumber(), nil
	default:
		return ConvertDatumToDecimal(ctx, *d)
	}
}

So we have to try to convert it explicitly.

Same for the float value eval process. We also need to convert it.

Because #8412 disabled the vectorized execution of get or set variable, so we don't really execute that code right now. But I also updated it.

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.

Fixed an issue that TiDB could not correctly convert the type of a session variable
修复 TiDB 无法正确转换表达式中系统变量类型的问题

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hi-rustin, XuHuaiyu

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• pkg/expression/OWNERS [XuHuaiyu]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-05-21 05:44:14.541891602 +0000 UTC m=+2150408.299027172: ☑️ agreed by hi-rustin.
• 2024-05-21 05:59:05.439336895 +0000 UTC m=+2151299.196472462: ☑️ agreed by XuHuaiyu.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 16 lines in your changes are missing coverage. Please review.

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Additional details and impacted files

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##             release-7.5     #53415   +/-   ##
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Components	Coverage Δ
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br	53.4010% <0.0000%> (?)

[qw4990]

/retest