expression: use the correct type when eval decimal and float session var

[Rustin170506]

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

[Rustin170506]

🔢 Self-check

cc: @time-and-fate

Thanks for your help!

[Rustin170506]

/retest

[codecov]

Codecov Report

Merging #51395 (a3484a5) into master (c23f0c9) will increase coverage by 2.4235%.
Report is 23 commits behind head on master.
The diff coverage is 20.0000%.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #51395        +/-   ##
================================================
+ Coverage   70.6663%   73.0898%   +2.4235%     
================================================
  Files          1461       1462         +1     
  Lines        434876     442198      +7322     
================================================
+ Hits         307311     323202     +15891     
+ Misses       108307      99059      -9248     
- Partials      19258      19937       +679     

Flag	Coverage Δ
integration	49.3792% <20.0000%> (?)
unit	70.6050% <20.0000%> (+0.1424%)	⬆️

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

Components	Coverage Δ
dumpling	53.9957% <ø> (ø)
parser	∅ <ø> (∅)
br	51.2558% <ø> (+5.3537%)	⬆️

[Rustin170506]

/retest

[elsa0520]

LGTM

[XuHuaiyu]

What about the implementation of (b *builtinGetRealVarSig) vecEvalReal

[zanmato1984]

+1

[Rustin170506]

Because this PR: #8412

There is no way to vectorized eval real. But I guess it makes sense to update it as well.

[zanmato1984]

Yes, please. Much appreciated.

Let's talk about enabling vectorization for get/set var function in the future.

[zanmato1984]

Is there any case we can add into this UT:

tidb/pkg/expression/builtin_other_test.go

Line 118 in 27ce02a

func TestGetVar(t *testing.T) {

.
As well as https://github.com/pingcap/tidb/blob/27ce02afd2e3d312b2bb92e20489cefce000e320/pkg/expression/builtin_other_vec_test.go , assuming you are going to add vectorized version of builtinGetRealVarSig.

[zanmato1984]

+1

[Rustin170506]

🔢 Self-check

[zanmato1984]

LGTM

[elsa0520]

LGTM

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AilinKid, elsa0520, zanmato1984

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• pkg/expression/OWNERS [zanmato1984]

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-02-28 09:20:18.283019794 +0000 UTC m=+1040107.030642913: ☑️ agreed by AilinKid.
• 2024-02-28 10:00:50.671372065 +0000 UTC m=+1042539.418995171: ☑️ agreed by elsa0520.
• 2024-02-29 03:41:45.72004397 +0000 UTC m=+1106194.467667080: ✖️🔁 reset by zanmato1984.
• 2024-02-29 09:18:32.645908407 +0000 UTC m=+1126401.393531518: ☑️ agreed by zanmato1984.
• 2024-02-29 09:34:25.804268372 +0000 UTC m=+1127354.551891483: ☑️ agreed by elsa0520.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #53415.