expression: Fixing Expression Rewrite Bug in Predicate Push Down for NULLIF with Type Mismatch

[dash12653]

What problem does this PR solve?

Issue Number: close #57647

Problem Summary:

What changed and how does it work?

The sql could be simplified as:

select *
from `t0`
where (
  nullif(3^1, case when (`c0`) then 91 end)
) in (select 1 from `t0`);

When rewriting the expression nullif(3^1, case when (c0) then 91 end), it is transformed into:
if(equals(3^1, case when (c0) then 91 end), null, 3^1).

When building this function, the RetType of null is inferred based on the type of 3^1, and a new RetType is set.

tidb/pkg/expression/builtin_control.go

Lines 713 to 720 in a82a3b4

	func (c *ifFunctionClass) getFunction(ctx BuildContext, args []Expression) (sig builtinFunc, err error) {
	if err = c.verifyArgs(args); err != nil {
	return nil, err
	}
	retTp, err := InferType4ControlFuncs(ctx, c.funcName, args[1], args[2])
	if err != nil {
	return nil, err
	}

Then, the SQL:

where (nullif(3^1, case when (c0) then 91 end)) in (select 1 from t0);
is rewritten to an inner join with otherConditions as:

nullif(3^1, case when (c0) then 91 end) = 1.

During predicate push down, it will try moving otherConditions into equal conditions:

tidb/pkg/planner/core/operator/logicalop/logical_join.go

Lines 1564 to 1572 in b11cce7

	for i := range leftKeys {
	lKey, rKey := leftKeys[i], rightKeys[i]
	if lProj != nil {
	lKey = lProj.AppendExpr(lKey)
	}
	if rProj != nil {
	rKey = rProj.AppendExpr(rKey)
	}
	eqCond := expression.NewFunctionInternal(p.SCtx().GetExprCtx(), ast.EQ, types.NewFieldType(mysql.TypeTiny), lKey, rKey)

But when rewriting the equal condition, the RetType of null has already been reset based on the earlier inference. So re-inferring of the RetType of null and 3^1 will not match previous result, and the type finally becomes decimal.

tidb/pkg/expression/builtin_control.go

Lines 279 to 289 in a82a3b4

	} else {
	resultFieldType = types.AggFieldType(notNullFields)
	var tempFlag uint
	evalType := types.AggregateEvalType(notNullFields, &tempFlag)
	resultFieldType.SetFlag(tempFlag)
	setDecimalFromArgs(evalType, resultFieldType, notNullFields...)
	err := addCollateAndCharsetAndFlagFromArgs(ctx, funcName, evalType, resultFieldType, args...)
	if err != nil {
	return nil, err
	}
	setFlenFromArgs(evalType, resultFieldType, notNullFields...)

The constant 1 will be wrapped with a (cast as decimal), making it no longer a column, which ultimately leads to the error.

I modified the ifInferType4ControlFuncs() method to determine whether a value is null. If a constant's value.k is null, it will also be treated as null.

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 73.5408%. Comparing base (068b9a8) to head (76b60c8).

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #58588        +/-   ##
================================================
+ Coverage   73.1052%   73.5408%   +0.4356%     
================================================
  Files          1676       1676                
  Lines        463369     463413        +44     
================================================
+ Hits         338747     340798      +2051     
+ Misses       103803     101772      -2031     
- Partials      20819      20843        +24     

Flag	Coverage Δ
integration	43.0337% <100.0000%> (?)
unit	72.2979% <100.0000%> (-0.0031%)	⬇️

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

Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.8023% <ø> (-0.0090%)	⬇️