By setting extra_float_digits to its maximal value (instead of the default 0)
we can ensure that the conversion from text (which we use when saving the
values in the database) to double (used internally) and back (when we retrieve
the values) is lossless and so that we read back exactly the same values as we
had written.

In principle, the best solution would be to use binary format with
PQexecParams() instead: this would avoid the conversion to/from text entirely
and so be undoubtedly correct and probably more efficient. However it also
would be more difficult, especially for NUMERIC types as we would need to
convert them to/from the protocol wire format ourselves, so for now just make
it work correctly in the simplest possible way.