select: add MISSING operator support (#14406)

Probably not full support, but for regular checks it should work.

Fixes #14358

internal/s3select/sql/value.go

@@ -49,6 +49,9 @@ type Value struct {
 	value interface{}
 }
 
+// Missing is used to indicate a non-existing value.
+type Missing struct{}
+
 // MarshalJSON provides json marshaling of values.
 func (v Value) MarshalJSON() ([]byte, error) {
 	if b, ok := v.ToBytes(); ok {
@@ -76,6 +79,8 @@ func (v Value) GetTypeString() string {
 		return "BYTES"
 	case []Value:
 		return "ARRAY"
+	case Missing:
+		return "MISSING"
 	}
 	return "--"
 }
@@ -104,6 +109,8 @@ func (v Value) Repr() string {
 		}
 		s.WriteString("]:ARRAY")
 		return s.String()
+	case Missing:
+		return ":MISSING"
 	default:
 		return fmt.Sprintf("%v:INVALID", v.value)
 	}
@@ -139,6 +146,11 @@ func FromNull() *Value {
 	return &Value{value: nil}
 }
 
+// FromMissing creates a Value with Missing value
+func FromMissing() *Value {
+	return &Value{value: Missing{}}
+}
+
 // FromBytes creates a Value from a []byte
 func FromBytes(b []byte) *Value {
 	return &Value{value: b}
@@ -239,6 +251,12 @@ func (v Value) IsNull() bool {
 	return false
 }
 
+// IsMissing - checks if value is missing.
+func (v Value) IsMissing() bool {
+	_, ok := v.value.(Missing)
+	return ok
+}
+
 // IsArray returns whether the value is an array.
 func (v Value) IsArray() (ok bool) {
 	_, ok = v.value.([]Value)
@@ -283,7 +301,7 @@ func (v Value) String() string {
 // CSVString - convert to string for CSV serialization
 func (v Value) CSVString() string {
 	switch x := v.value.(type) {
-	case nil:
+	case nil, Missing:
 		return ""
 	case bool:
 		if x {
@@ -390,13 +408,21 @@ func (v *Value) compareOp(op string, a *Value) (res bool, err error) {
 	switch op {
 	case opIs:
 		if a.IsNull() {
-			return v.IsNull(), nil
+			// Missing is null
+			return v.IsNull() || v.IsMissing(), nil
+		}
+		if a.IsMissing() {
+			return v.IsMissing(), nil
 		}
 		// Forward to Equal
 		op = opEq
 	case opIsNot:
 		if a.IsNull() {
-			return !v.IsNull(), nil
+			// Missing is not null
+			return !v.IsNull() && !v.IsMissing(), nil
+		}
+		if a.IsMissing() {
+			return !v.IsMissing(), nil
 		}
 		// Forward to not equal.
 		op = opIneq
@@ -416,6 +442,11 @@ func (v *Value) compareOp(op string, a *Value) (res bool, err error) {
 		return boolCompare(op, v.IsNull(), a.IsNull())
 	}
 
+	if a.IsMissing() || v.IsMissing() {
+		// If one is, both must be.
+		return boolCompare(op, v.IsMissing(), a.IsMissing())
+	}
+
 	// Check array values
 	aArr, aOK := a.ToArray()
 	vArr, vOK := v.ToArray()