func (a *varianceAggregate) add(datum parser.Datum) error {
	if datum == parser.DNull {
		return nil
	}

	var d parser.DFloat
	switch t := datum.(type) {
	case parser.DInt:
		d = parser.DFloat(t)
	case parser.DFloat:
		d = t
	// case parser.DDecimal:
	// TODO(nvanbenschoten) add support for decimal variance and stddev
	// aggregation functions. Will require adding decimal.Sqrt() to library.
	default:
		return util.Errorf("unexpected VARIANCE argument type: %s", datum.Type())
	}

	// Uses the Knuth/Welford method for accurately computing variance online in a
	// single pass. See http://www.johndcook.com/blog/standard_deviation/ and
	// https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Online_algorithm.
	a.count++
	delta := d - a.mean
	a.mean += delta / parser.DFloat(a.count)
	a.sqrDiff += delta * (d - a.mean)
	return nil
}

func decodeTableKey(valType parser.Datum, key []byte) (parser.Datum, []byte, error) {
	var isNull bool
	if key, isNull = encoding.DecodeIfNull(key); isNull {
		return parser.DNull, key, nil
	}
	switch valType.(type) {
	case parser.DBool:
		rkey, i, err := encoding.DecodeVarint(key)
		return parser.DBool(i != 0), rkey, err
	case parser.DInt:
		rkey, i, err := encoding.DecodeVarint(key)
		return parser.DInt(i), rkey, err
	case parser.DFloat:
		rkey, f, err := encoding.DecodeFloat(key, nil)
		return parser.DFloat(f), rkey, err
	case parser.DString:
		rkey, r, err := encoding.DecodeString(key, nil)
		return parser.DString(r), rkey, err
	case parser.DBytes:
		rkey, r, err := encoding.DecodeString(key, nil)
		return parser.DBytes(r), rkey, err
	case parser.DDate:
		rkey, t, err := encoding.DecodeTime(key)
		return parser.DDate{Time: t}, rkey, err
	case parser.DTimestamp:
		rkey, t, err := encoding.DecodeTime(key)
		return parser.DTimestamp{Time: t}, rkey, err
	case parser.DInterval:
		rkey, d, err := encoding.DecodeVarint(key)
		return parser.DInterval{Duration: time.Duration(d)}, rkey, err
	default:
		return nil, nil, util.Errorf("TODO(pmattis): decoded index key: %s", valType.Type())
	}
}

func datumFromProto(d driver.Datum) parser.Datum {
	arg := d.Payload
	if arg == nil {
		return parser.DNull
	}
	switch t := arg.(type) {
	case *driver.Datum_BoolVal:
		return parser.DBool(t.BoolVal)
	case *driver.Datum_IntVal:
		return parser.DInt(t.IntVal)
	case *driver.Datum_FloatVal:
		return parser.DFloat(t.FloatVal)
	case *driver.Datum_DecimalVal:
		dd := &parser.DDecimal{}
		if _, ok := dd.SetString(t.DecimalVal); !ok {
			panic(fmt.Sprintf("could not parse string %q as decimal", t.DecimalVal))
		}
		return dd
	case *driver.Datum_BytesVal:
		return parser.DBytes(t.BytesVal)
	case *driver.Datum_StringVal:
		return parser.DString(t.StringVal)
	case *driver.Datum_DateVal:
		return parser.DDate(t.DateVal)
	case *driver.Datum_TimeVal:
		return parser.DTimestamp{Time: t.TimeVal.GoTime()}
	case *driver.Datum_IntervalVal:
		return parser.DInterval{Duration: time.Duration(t.IntervalVal)}
	default:
		panic(fmt.Sprintf("unexpected type %T", t))
	}
}

// decodeKeyVals decodes the values that are part of the key. ValTypes is a
// slice returned from makeKeyVals. The decoded values are stored in the vals
// parameter while the valTypes parameter is unmodified. Note that len(vals) >=
// len(valTypes). The types of the decoded values will match the corresponding
// entry in the valTypes parameter with the exception that a value might also
// be parser.DNull. The remaining bytes in the key after decoding the values
// are returned.
func decodeKeyVals(valTypes, vals []parser.Datum, key []byte) ([]byte, error) {
	for j := range valTypes {
		var isNull bool
		if key, isNull = encoding.DecodeIfNull(key); isNull {
			vals[j] = parser.DNull
			continue
		}

		switch valTypes[j].(type) {
		case parser.DInt:
			var i int64
			key, i = encoding.DecodeVarint(key)
			vals[j] = parser.DInt(i)
		case parser.DFloat:
			var f float64
			key, f = encoding.DecodeFloat(key, nil)
			vals[j] = parser.DFloat(f)
		case parser.DString:
			var r string
			key, r = encoding.DecodeString(key, nil)
			vals[j] = parser.DString(r)
		default:
			return nil, util.Errorf("TODO(pmattis): decoded index key: %s", valTypes[j].Type())
		}
	}

	return key, nil
}

// Arg implements the parser.Args interface.
func (p parameters) Arg(name string) (parser.Datum, bool) {
	i, err := processPositionalArgument(name)
	if err != nil {
		return nil, false
	}
	if i < 1 || int(i) > len(p) {
		return nil, false
	}
	arg := p[i-1].Payload
	if arg == nil {
		return parser.DNull, true
	}
	switch t := arg.(type) {
	case *driver.Datum_BoolVal:
		return parser.DBool(t.BoolVal), true
	case *driver.Datum_IntVal:
		return parser.DInt(t.IntVal), true
	case *driver.Datum_FloatVal:
		return parser.DFloat(t.FloatVal), true
	case *driver.Datum_BytesVal:
		return parser.DBytes(t.BytesVal), true
	case *driver.Datum_StringVal:
		return parser.DString(t.StringVal), true
	case *driver.Datum_DateVal:
		return parser.DDate(t.DateVal), true
	case *driver.Datum_TimeVal:
		return parser.DTimestamp{Time: t.TimeVal.GoTime()}, true
	case *driver.Datum_IntervalVal:
		return parser.DInterval{Duration: time.Duration(t.IntervalVal)}, true
	default:
		panic(fmt.Sprintf("unexpected type %T", t))
	}
}

func decodeTableKey(valType parser.Datum, key []byte) (parser.Datum, []byte, error) {
	var isNull bool
	if key, isNull = encoding.DecodeIfNull(key); isNull {
		return parser.DNull, key, nil
	}
	switch valType.(type) {
	case parser.DBool:
		var i int64
		key, i = encoding.DecodeVarint(key)
		return parser.DBool(i != 0), key, nil
	case parser.DInt:
		var i int64
		key, i = encoding.DecodeVarint(key)
		return parser.DInt(i), key, nil
	case parser.DFloat:
		var f float64
		key, f = encoding.DecodeFloat(key, nil)
		return parser.DFloat(f), key, nil
	case parser.DString:
		var r string
		key, r = encoding.DecodeString(key, nil)
		return parser.DString(r), key, nil
	default:
		return nil, nil, util.Errorf("TODO(pmattis): decoded index key: %s", valType.Type())
	}
}

func datumFromProto(d driver.Datum) parser.Datum {
	arg := d.Payload
	if arg == nil {
		return parser.DNull
	}
	switch t := arg.(type) {
	case *driver.Datum_BoolVal:
		return parser.DBool(t.BoolVal)
	case *driver.Datum_IntVal:
		return parser.DInt(t.IntVal)
	case *driver.Datum_FloatVal:
		return parser.DFloat(t.FloatVal)
	case *driver.Datum_DecimalVal:
		dec, err := decimal.NewFromString(t.DecimalVal)
		if err != nil {
			panic(fmt.Sprintf("could not parse decimal: %v", err))
		}
		return parser.DDecimal{Decimal: dec}
	case *driver.Datum_BytesVal:
		return parser.DBytes(t.BytesVal)
	case *driver.Datum_StringVal:
		return parser.DString(t.StringVal)
	case *driver.Datum_DateVal:
		return parser.DDate(t.DateVal)
	case *driver.Datum_TimeVal:
		return parser.DTimestamp{Time: t.TimeVal.GoTime()}
	case *driver.Datum_IntervalVal:
		return parser.DInterval{Duration: time.Duration(t.IntervalVal)}
	default:
		panic(fmt.Sprintf("unexpected type %T", t))
	}
}

// Arg implements the Args interface
func (p parameters) Arg(name string) (parser.Datum, bool) {
	if !unicode.IsDigit(rune(name[0])) {
		// TODO(pmattis): Add support for named parameters (vs the numbered
		// parameter support below).
		return nil, false
	}
	i, err := strconv.ParseInt(name, 10, 0)
	if err != nil {
		return nil, false
	}
	if i < 1 || int(i) > len(p) {
		return nil, false
	}
	arg := p[i-1].GetValue()
	if arg == nil {
		return parser.DNull, true
	}
	switch t := arg.(type) {
	case *bool:
		return parser.DBool(*t), true
	case *int64:
		return parser.DInt(*t), true
	case *float64:
		return parser.DFloat(*t), true
	case []byte:
		return parser.DString(t), true
	case *string:
		return parser.DString(*t), true
	default:
		panic(fmt.Sprintf("unexpected type %T", t))
	}
}

func (n *scanNode) unmarshalValue(kv client.KeyValue) (parser.Datum, bool) {
	kind, ok := n.colKind[n.colID]
	if !ok {
		n.err = fmt.Errorf("column-id \"%d\" does not exist", n.colID)
		return nil, false
	}
	if kv.Exists() {
		switch kind {
		case ColumnType_INT:
			return parser.DInt(kv.ValueInt()), true
		case ColumnType_BOOL:
			return parser.DBool(kv.ValueInt() != 0), true
		case ColumnType_FLOAT:
			return parser.DFloat(math.Float64frombits(uint64(kv.ValueInt()))), true
		case ColumnType_STRING, ColumnType_BYTES:
			return parser.DString(kv.ValueBytes()), true
		case ColumnType_DATE:
			var t time.Time
			if err := t.UnmarshalBinary(kv.ValueBytes()); err != nil {
				return nil, false
			}
			return parser.DDate{Time: t}, true
		case ColumnType_TIMESTAMP:
			var t time.Time
			if err := t.UnmarshalBinary(kv.ValueBytes()); err != nil {
				return nil, false
			}
			return parser.DTimestamp{Time: t}, true
		case ColumnType_INTERVAL:
			return parser.DInterval{Duration: time.Duration(kv.ValueInt())}, true
		}
	}
	return parser.DNull, true
}

func makeIndexKeyVals(desc *structured.TableDescriptor,
	index structured.IndexDescriptor) ([]parser.Datum, error) {
	vals := make([]parser.Datum, len(index.ColumnIDs))
	for i, id := range index.ColumnIDs {
		col, err := desc.FindColumnByID(id)
		if err != nil {
			return nil, err
		}
		switch col.Type.Kind {
		case structured.ColumnType_BIT, structured.ColumnType_INT:
			vals[i] = parser.DInt(0)
		case structured.ColumnType_FLOAT:
			vals[i] = parser.DFloat(0)
		case structured.ColumnType_CHAR, structured.ColumnType_TEXT,
			structured.ColumnType_BLOB:
			vals[i] = parser.DString("")
		default:
			return nil, util.Errorf("TODO(pmattis): decoded index key: %s", col.Type.Kind)
		}
	}
	if !index.Unique {
		// Non-unique columns are suffixed by the primary index key.
		pkVals, err := makeIndexKeyVals(desc, desc.PrimaryIndex)
		if err != nil {
			return nil, err
		}
		vals = append(vals, pkVals...)
	}
	return vals, nil
}

func (n *scanNode) getQVal(col ColumnDescriptor) parser.Expr {
	if n.qvals == nil {
		n.qvals = make(qvalMap)
	}
	qval := n.qvals[col.ID]
	if qval == nil {
		qval = &qvalue{col: col}
		// We initialize the qvalue expression to a datum of the type matching the
		// column. This allows type analysis to be performed on the expression
		// before we start retrieving rows.
		//
		// TODO(pmattis): Nullable columns can have NULL values. The type analysis
		// needs to take that into consideration, but how to surface that info?
		switch col.Type.Kind {
		case ColumnType_BIT, ColumnType_INT:
			qval.datum = parser.DInt(0)
		case ColumnType_BOOL:
			qval.datum = parser.DBool(true)
		case ColumnType_FLOAT:
			qval.datum = parser.DFloat(0)
		case ColumnType_CHAR, ColumnType_TEXT,
			ColumnType_BLOB:
			qval.datum = parser.DString("")
		default:
			panic(fmt.Sprintf("unsupported column type: %s", col.Type.Kind))
		}
		n.qvals[col.ID] = qval
	}
	return qval
}

func (a *avgAggregate) Result() (parser.Datum, error) {
	sum, err := a.sumAggregate.Result()
	if err != nil {
		return parser.DNull, err
	}
	if sum == parser.DNull {
		return sum, nil
	}
	switch t := sum.(type) {
	case parser.DInt:
		return parser.DFloat(t) / parser.DFloat(a.count), nil
	case parser.DFloat:
		return t / parser.DFloat(a.count), nil
	default:
		return parser.DNull, fmt.Errorf("unexpected SUM result type: %s", t.Type())
	}
}

// DecodeTableValue decodes a value encoded by EncodeTableValue.
func DecodeTableValue(a *DatumAlloc, valType parser.Datum, b []byte) (parser.Datum, []byte, error) {
	// TODO(dan): Merge this and DecodeTableKey.
	if len(b) == 0 {
		return nil, nil, util.Errorf("empty slice")
	}
	if roachpb.ValueType(b[0]) == roachpb.ValueType_NULL {
		return parser.DNull, b[1:], nil
	}
	var err error
	switch valType.(type) {
	case *parser.DBool:
		var i int64
		b, i, err = roachpb.DecodeIntValue(b)
		// No need to chunk allocate DBool as MakeDBool returns either
		// parser.DBoolTrue or parser.DBoolFalse.
		return parser.MakeDBool(parser.DBool(i != 0)), b, err
	case *parser.DInt:
		var i int64
		b, i, err = roachpb.DecodeIntValue(b)
		return a.NewDInt(parser.DInt(i)), b, err
	case *parser.DFloat:
		var f float64
		b, f, err = roachpb.DecodeFloatValue(b)
		return a.NewDFloat(parser.DFloat(f)), b, err
	case *parser.DDecimal:
		var d *inf.Dec
		b, d, err = roachpb.DecodeDecimalValue(b)
		dd := a.NewDDecimal(parser.DDecimal{})
		dd.Set(d)
		return dd, b, err
	case *parser.DString:
		var data []byte
		b, data, err = roachpb.DecodeBytesValue(b)
		return a.NewDString(parser.DString(data)), b, err
	case *parser.DBytes:
		var data []byte
		b, data, err = roachpb.DecodeBytesValue(b)
		return a.NewDBytes(parser.DBytes(data)), b, err
	case *parser.DDate:
		var i int64
		b, i, err = roachpb.DecodeIntValue(b)
		return a.NewDDate(parser.DDate(i)), b, err
	case *parser.DTimestamp:
		var t time.Time
		b, t, err = roachpb.DecodeTimeValue(b)
		return a.NewDTimestamp(parser.DTimestamp{Time: t}), b, err
	case *parser.DTimestampTZ:
		var t time.Time
		b, t, err = roachpb.DecodeTimeValue(b)
		return a.NewDTimestampTZ(parser.DTimestampTZ{Time: t}), b, err
	case *parser.DInterval:
		var d duration.Duration
		b, d, err = roachpb.DecodeDurationValue(b)
		return a.NewDInterval(parser.DInterval{Duration: d}), b, err
	default:
		return nil, nil, util.Errorf("TODO(pmattis): decoded index value: %s", valType.Type())
	}
}

// unmarshalColumnValue decodes the value from a key-value pair using the type
// expected by the column. An error is returned if the value's type does not
// match the column's type.
func unmarshalColumnValue(kind ColumnType_Kind, value *proto.Value) (parser.Datum, error) {
	if value == nil {
		return parser.DNull, nil
	}

	switch kind {
	case ColumnType_BOOL:
		v, err := value.GetInt()
		if err != nil {
			return nil, err
		}
		return parser.DBool(v != 0), nil
	case ColumnType_INT:
		v, err := value.GetInt()
		if err != nil {
			return nil, err
		}
		return parser.DInt(v), nil
	case ColumnType_FLOAT:
		v, err := value.GetFloat()
		if err != nil {
			return nil, err
		}
		return parser.DFloat(v), nil
	case ColumnType_STRING:
		v, err := value.GetBytesChecked()
		if err != nil {
			return nil, err
		}
		return parser.DString(v), nil
	case ColumnType_BYTES:
		v, err := value.GetBytesChecked()
		if err != nil {
			return nil, err
		}
		return parser.DBytes(v), nil
	case ColumnType_DATE:
		v, err := value.GetTime()
		if err != nil {
			return nil, err
		}
		return parser.DDate{Time: v}, nil
	case ColumnType_TIMESTAMP:
		v, err := value.GetTime()
		if err != nil {
			return nil, err
		}
		return parser.DTimestamp{Time: v}, nil
	case ColumnType_INTERVAL:
		v, err := value.GetInt()
		if err != nil {
			return nil, err
		}
		return parser.DInterval{Duration: time.Duration(v)}, nil
	default:
		return nil, util.Errorf("unsupported column type: %s", kind)
	}
}

// DecodeTableValue decodes a value encoded by EncodeTableValue.
func DecodeTableValue(a *DatumAlloc, valType parser.Datum, b []byte) (parser.Datum, []byte, error) {
	_, dataOffset, _, typ, err := encoding.DecodeValueTag(b)
	if err != nil {
		return nil, b, err
	}
	if typ == encoding.Null {
		return parser.DNull, b[dataOffset:], nil
	}
	switch valType.(type) {
	case *parser.DBool:
		var x bool
		b, x, err = encoding.DecodeBoolValue(b)
		// No need to chunk allocate DBool as MakeDBool returns either
		// parser.DBoolTrue or parser.DBoolFalse.
		return parser.MakeDBool(parser.DBool(x)), b, err
	case *parser.DInt:
		var i int64
		b, i, err = encoding.DecodeIntValue(b)
		return a.NewDInt(parser.DInt(i)), b, err
	case *parser.DFloat:
		var f float64
		b, f, err = encoding.DecodeFloatValue(b)
		return a.NewDFloat(parser.DFloat(f)), b, err
	case *parser.DDecimal:
		var d *inf.Dec
		b, d, err = encoding.DecodeDecimalValue(b)
		dd := a.NewDDecimal(parser.DDecimal{})
		dd.Set(d)
		return dd, b, err
	case *parser.DString:
		var data []byte
		b, data, err = encoding.DecodeBytesValue(b)
		return a.NewDString(parser.DString(data)), b, err
	case *parser.DBytes:
		var data []byte
		b, data, err = encoding.DecodeBytesValue(b)
		return a.NewDBytes(parser.DBytes(data)), b, err
	case *parser.DDate:
		var i int64
		b, i, err = encoding.DecodeIntValue(b)
		return a.NewDDate(parser.DDate(i)), b, err
	case *parser.DTimestamp:
		var t time.Time
		b, t, err = encoding.DecodeTimeValue(b)
		return a.NewDTimestamp(parser.DTimestamp{Time: t}), b, err
	case *parser.DTimestampTZ:
		var t time.Time
		b, t, err = encoding.DecodeTimeValue(b)
		return a.NewDTimestampTZ(parser.DTimestampTZ{Time: t}), b, err
	case *parser.DInterval:
		var d duration.Duration
		b, d, err = encoding.DecodeDurationValue(b)
		return a.NewDInterval(parser.DInterval{Duration: d}), b, err
	default:
		return nil, nil, errors.Errorf("TODO(pmattis): decoded index value: %s", valType.Type())
	}
}

// golangFillQueryArguments populates the placeholder map with
// types and values from an array of Go values.
// TODO: This does not support arguments of the SQL 'Date' type, as there is not
// an equivalent type in Go's standard library. It's not currently needed by any
// of our internal tables.
func golangFillQueryArguments(pinfo *parser.PlaceholderInfo, args []interface{}) {
	pinfo.Clear()

	for i, arg := range args {
		k := fmt.Sprint(i + 1)
		if arg == nil {
			pinfo.SetValue(k, parser.DNull)
			continue
		}

		// A type switch to handle a few explicit types with special semantics:
		// - Datums are passed along as is.
		// - Time datatypes get special representation in the database.
		var d parser.Datum
		switch t := arg.(type) {
		case parser.Datum:
			d = t
		case time.Time:
			d = parser.MakeDTimestamp(t, time.Microsecond)
		case time.Duration:
			d = &parser.DInterval{Duration: duration.Duration{Nanos: t.Nanoseconds()}}
		case *inf.Dec:
			dd := &parser.DDecimal{}
			dd.Set(t)
			d = dd
		}
		if d == nil {
			// Handle all types which have an underlying type that can be stored in the
			// database.
			// Note: if this reflection becomes a performance concern in the future,
			// commonly used types could be added explicitly into the type switch above
			// for a performance gain.
			val := reflect.ValueOf(arg)
			switch val.Kind() {
			case reflect.Bool:
				d = parser.MakeDBool(parser.DBool(val.Bool()))
			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
				d = parser.NewDInt(parser.DInt(val.Int()))
			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
				d = parser.NewDInt(parser.DInt(val.Uint()))
			case reflect.Float32, reflect.Float64:
				d = parser.NewDFloat(parser.DFloat(val.Float()))
			case reflect.String:
				d = parser.NewDString(val.String())
			case reflect.Slice:
				// Handle byte slices.
				if val.Type().Elem().Kind() == reflect.Uint8 {
					d = parser.NewDBytes(parser.DBytes(val.Bytes()))
				}
			}
			if d == nil {
				panic(fmt.Sprintf("unexpected type %T", arg))
			}
		}
		pinfo.SetValue(k, d)
	}
}

func makeFloatTestDatum(count int) []parser.Datum {
	rng, _ := randutil.NewPseudoRand()

	vals := make([]parser.Datum, count)
	for i := range vals {
		vals[i] = parser.DFloat(rng.Float64())
	}
	return vals
}

// Arg implements the parser.Args interface.
// TODO: This does not support arguments of the SQL 'Date' type, as there is not
// an equivalent type in Go's standard library. It's not currently needed by any
// of our internal tables.
func (gp golangParameters) Arg(name string) (parser.Datum, bool) {
	i, err := processPositionalArgument(name)
	if err != nil {
		return nil, false
	}
	if i < 1 || int(i) > len(gp) {
		return nil, false
	}
	arg := gp[i-1]
	if arg == nil {
		return parser.DNull, true
	}

	// A type switch to handle a few explicit types with special semantics.
	switch t := arg.(type) {
	// Datums are passed along as is.
	case parser.Datum:
		return t, true
	// Time datatypes get special representation in the database.
	case time.Time:
		return parser.DTimestamp{Time: t}, true
	case time.Duration:
		return parser.DInterval{Duration: t}, true
	case *inf.Dec:
		dd := &parser.DDecimal{}
		dd.Set(t)
		return dd, true
	}

	// Handle all types which have an underlying type that can be stored in the
	// database.
	// Note: if this reflection becomes a performance concern in the future,
	// commonly used types could be added explicitly into the type switch above
	// for a performance gain.
	val := reflect.ValueOf(arg)
	switch val.Kind() {
	case reflect.Bool:
		return parser.DBool(val.Bool()), true
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return parser.DInt(val.Int()), true
	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
		return parser.DInt(val.Uint()), true
	case reflect.Float32, reflect.Float64:
		return parser.DFloat(val.Float()), true
	case reflect.String:
		return parser.DString(val.String()), true
	case reflect.Slice:
		// Handle byte slices.
		if val.Type().Elem().Kind() == reflect.Uint8 {
			return parser.DBytes(val.Bytes()), true
		}
	}

	panic(fmt.Sprintf("unexpected type %T", arg))
}

func (a *stddevAggregate) result() (parser.Datum, error) {
	variance, err := a.varianceAggregate.result()
	if err != nil || variance == parser.DNull {
		return variance, err
	}
	switch t := variance.(type) {
	case parser.DFloat:
		return parser.DFloat(math.Sqrt(float64(t))), nil
	}
	return nil, util.Errorf("unexpected variance result type: %s", variance.Type())
}

func (a *avgAggregate) result() (parser.Datum, error) {
	sum, err := a.sumAggregate.result()
	if err != nil {
		return parser.DNull, err
	}
	if sum == parser.DNull {
		return sum, nil
	}
	switch t := sum.(type) {
	case parser.DInt:
		// TODO(nvanbenschoten) decimal: this should be a numeric, once
		// better type coercion semantics are defined.
		return parser.DFloat(t) / parser.DFloat(a.count), nil
	case parser.DFloat:
		return t / parser.DFloat(a.count), nil
	case parser.DDecimal:
		return parser.DDecimal{Decimal: t.Div(decimal.New(int64(a.count), 0))}, nil
	default:
		return parser.DNull, util.Errorf("unexpected SUM result type: %s", t.Type())
	}
}