Function EncodeValueTag

pkg/util/encoding/encoding.go:2568–2580

EncodeValueTag encodes the prefix that is used by each of the EncodeFooValue methods. The prefix uses varints to encode a column id and type, packing them into a single byte when they're small (colIDDelta < 8 and typ < 15). This works by shifting the colIDDelta "left" by 4 and putting any type less

(appendTo []byte, colIDDelta uint32, typ Type)

// Together, this means the everything but the last byte of the first uvarint
// can be dropped if the column id isn't needed.
func EncodeValueTag(appendTo []byte, colIDDelta uint32, typ Type) []byte {
	if typ >= SentinelType {
		appendTo = EncodeNonsortingUvarint(appendTo, uint64(colIDDelta)<<4|uint64(SentinelType))
		return EncodeNonsortingUvarint(appendTo, uint64(typ))
	}
	if colIDDelta == NoColumnID {
		// TODO(dan): EncodeValueTag is not inlined by the compiler. Copying this
		// special case into one of the EncodeFooValue functions speeds it up by
		// ~4ns.
		return append(appendTo, byte(typ))
	}
	return EncodeNonsortingUvarint(appendTo, uint64(colIDDelta)<<4|uint64(typ))
}

// EncodeNullValue encodes a null value, appends it to the supplied buffer, and
// returns the final buffer.