Function text_preserves_uniqueness 

pub(crate) fn text_preserves_uniqueness(ty: &SqlScalarType) -> bool

Reports whether rendering values of type ty as text maps distinct values to distinct strings.

Values of time, timestamp and timestamptz can carry nanoseconds, for example from date + time, but their text form is rounded to microseconds. Any type that contains one of them is therefore not uniqueness-preserving.