ORCA: Fix eliminate self comparison

For the below query

create table t1(a int, b int not null);
create table t2(like t1);
select t1.*, t2.* from t1 full join t2 on false where (t1.b < t1.b) is
null;

orca generates a wrong plan:

Result  (cost=0.00..0.00 rows=0 width=16)
    One-Time Filter: false
Optimizer: Pivotal Optimizer (GPORCA)

The root cause is '(t1.b < t1.b)' is been transformed into
'CScalarConst (0)' by 'PexprEliminateSelfComparison'.
The reason is that when checking if the `selfcomparison` can be
simplified by function `FSelfComparison`, it checks the `CColRef`
IsNullable only from the column definition, not checking if the column
is from outer join.

To fix it, before simplifing the scalar expression, we fisrt get
the 'pcrsNotNull' from its parent expression. 'pcrsNotNull' recoreds
the output columns' nullable property. If the column is not in
'pcrsNotNull', then the self comparison cannot be transformed into
const true or false.

src/backend/gporca/libgpopt/include/gpopt/operators/CExpressionPreprocessor.h

@@ -70,7 +70,8 @@ class CExpressionPreprocessor
 
 	// eliminate self comparisons
 	static CExpression *PexprEliminateSelfComparison(CMemoryPool *mp,
-													 CExpression *pexpr);
+													 CExpression *pexpr,
+													 CColRefSet *pcrsNotNull);
 
 	// remove CTE Anchor nodes
 	static CExpression *PexprRemoveCTEAnchors(CMemoryPool *mp,

src/backend/gporca/libgpopt/include/gpopt/operators/CPredicateUtils.h

@@ -130,11 +130,14 @@ class CPredicateUtils
 	static BOOL FPlainEquality(CExpression *pexpr);
 
 	// is the given expression a self comparison on some column
-	static BOOL FSelfComparison(CExpression *pexpr, IMDType::ECmpType *pecmpt);
+	static BOOL FSelfComparison(CExpression *pexpr,
+							IMDType::ECmpType *pecmpt,
+							CColRefSet *pcrsNotNull);
 
 	// eliminate self comparison if possible
 	static CExpression *PexprEliminateSelfComparison(CMemoryPool *mp,
-													 CExpression *pexpr);
+													 CExpression *pexpr,
+													 CColRefSet *pcrsNotNull);
 
 	// is the given expression in the form (col1 Is NOT DISTINCT FROM col2)
 	static BOOL FINDFScalarIdents(CExpression *pexpr);

src/backend/gporca/libgpopt/src/operators/CExpressionPreprocessor.cpp

@@ -70,29 +70,31 @@ using namespace gpopt;
 // eliminate self comparisons in the given expression
 CExpression *
 CExpressionPreprocessor::PexprEliminateSelfComparison(CMemoryPool *mp,
-													  CExpression *pexpr)
+													  CExpression *pexpr,
+													  CColRefSet *pcrsNotNull)
 {
 	// protect against stack overflow during recursion
 	GPOS_CHECK_STACK_SIZE;
 	GPOS_ASSERT(nullptr != mp);
 	GPOS_ASSERT(nullptr != pexpr);
-
+	COperator *pop = pexpr->Pop();
 	if (CUtils::FScalarCmp(pexpr))
 	{
-		return CPredicateUtils::PexprEliminateSelfComparison(mp, pexpr);
+		return CPredicateUtils::PexprEliminateSelfComparison(mp, pexpr, pcrsNotNull);
+	}
+	if (pop->FLogical())
+	{
+		pcrsNotNull = pexpr->DeriveNotNullColumns();
 	}
-
 	// recursively process children
 	const ULONG arity = pexpr->Arity();
 	CExpressionArray *pdrgpexprChildren = GPOS_NEW(mp) CExpressionArray(mp);
 	for (ULONG ul = 0; ul < arity; ul++)
 	{
-		CExpression *pexprChild =
-			PexprEliminateSelfComparison(mp, (*pexpr)[ul]);
-		pdrgpexprChildren->Append(pexprChild);
+		pdrgpexprChildren->Append(
+			PexprEliminateSelfComparison(mp, (*pexpr)[ul], pcrsNotNull));
 	}
 
-	COperator *pop = pexpr->Pop();
 	pop->AddRef();
 
 	return GPOS_NEW(mp) CExpression(mp, pop, pdrgpexprChildren);
@@ -3078,7 +3080,7 @@ CExpressionPreprocessor::PexprPreprocess(
 
 	// (11) eliminate self comparisons
 	CExpression *pexprSelfCompEliminated =
-		PexprEliminateSelfComparison(mp, pexprInferredPreds);
+		PexprEliminateSelfComparison(mp, pexprInferredPreds, nullptr);
 	GPOS_CHECK_ABORT;
 	pexprInferredPreds->Release();
 

src/backend/gporca/libgpopt/src/operators/CPredicateUtils.cpp

@@ -856,7 +856,7 @@ CPredicateUtils::FPlainEquality(CExpression *pexpr)
 
 // is an expression a self comparison on some column
 BOOL
-CPredicateUtils::FSelfComparison(CExpression *pexpr, IMDType::ECmpType *pecmpt)
+CPredicateUtils::FSelfComparison(CExpression *pexpr, IMDType::ECmpType *pecmpt, CColRefSet *pcrsNotNull)
 {
 	GPOS_ASSERT(nullptr != pexpr);
 	GPOS_ASSERT(nullptr != pecmpt);
@@ -881,6 +881,10 @@ CPredicateUtils::FSelfComparison(CExpression *pexpr, IMDType::ECmpType *pecmpt)
 
 		CColRef *colref =
 			const_cast<CColRef *>(CScalarIdent::PopConvert(popLeft)->Pcr());
+		if (nullptr != pcrsNotNull && !pcrsNotNull->FMember(colref))
+		{
+			return false;
+		}
 
 		return CColRef::EcrtTable == colref->Ecrt() &&
 			   !CColRefTable::PcrConvert(colref)->IsNullable();
@@ -892,14 +896,15 @@ CPredicateUtils::FSelfComparison(CExpression *pexpr, IMDType::ECmpType *pecmpt)
 // eliminate self comparison and replace it with True or False if possible
 CExpression *
 CPredicateUtils::PexprEliminateSelfComparison(CMemoryPool *mp,
-											  CExpression *pexpr)
+											  CExpression *pexpr,
+											  CColRefSet *pcrsNotNull)
 {
 	GPOS_ASSERT(pexpr->Pop()->FScalar());
 
 	pexpr->AddRef();
 	CExpression *pexprNew = pexpr;
 	IMDType::ECmpType cmp_type = IMDType::EcmptOther;
-	if (FSelfComparison(pexpr, &cmp_type))
+	if (FSelfComparison(pexpr, &cmp_type, pcrsNotNull))
 	{
 		switch (cmp_type)
 		{