The digest of the selection represented by the implementation. The
selections that accompany Tables can modify the contained dataset, e.g.
a file selection can restrict to a subset of the available data and a
format selection can include options that affect the behaviour of the
underlying reader. Two scans will end up being considered identical during
logical planning if their digests are the same so selection
implementations should override this method so that exactly those scans
that really are identical (in terms of the data they produce) have matching