Function FoldComparisonExpr

postgres/parser/sem/tree/eval.go:1634–1672

FoldComparisonExpr folds a given comparison operation and its expressions into an equivalent operation that will hit in the CmpOps map, returning this new operation, along with potentially flipped operands and "flipped" and "not" flags.

(
	op ComparisonOperator, left, right Expr,
)

// this new operation, along with potentially flipped operands and "flipped"
// and "not" flags.
func FoldComparisonExpr(
	op ComparisonOperator, left, right Expr,
) (newOp ComparisonOperator, newLeft Expr, newRight Expr, flipped bool, not bool) {
	switch op {
	case NE:
		// NE(left, right) is implemented as !EQ(left, right).
		return EQ, left, right, false, true
	case GT:
		// GT(left, right) is implemented as LT(right, left)
		return LT, right, left, true, false
	case GE:
		// GE(left, right) is implemented as LE(right, left)
		return LE, right, left, true, false
	case NotIn:
		// NotIn(left, right) is implemented as !IN(left, right)
		return In, left, right, false, true
	case NotLike:
		// NotLike(left, right) is implemented as !Like(left, right)
		return Like, left, right, false, true
	case NotILike:
		// NotILike(left, right) is implemented as !ILike(left, right)
		return ILike, left, right, false, true
	case NotSimilarTo:
		// NotSimilarTo(left, right) is implemented as !SimilarTo(left, right)
		return SimilarTo, left, right, false, true
	case NotRegMatch:
		// NotRegMatch(left, right) is implemented as !RegMatch(left, right)
		return RegMatch, left, right, false, true
	case NotRegIMatch:
		// NotRegIMatch(left, right) is implemented as !RegIMatch(left, right)
		return RegIMatch, left, right, false, true
	case IsDistinctFrom:
		// IsDistinctFrom(left, right) is implemented as !IsNotDistinctFrom(left, right)
		// Note: this seems backwards, but IS NOT DISTINCT FROM is an extended
		// version of IS and IS DISTINCT FROM is an extended version of IS NOT.
		return IsNotDistinctFrom, left, right, false, true
	}
	return op, left, right, false, false
}