Re: What are the characteristics of a good user-defined data type?

"Tim Hart" <tjhart@mac.com> writes:
> I've reviewed the PostgreSQL documentation on user defined data types. It
> seems that data types that can't be ordered or compared for equality would
> be bad candidates. After all, if a data type can't be indexed or used in a
> where clause, what value does a custom type bring over a binary or textual
> representation?

Well, the possibility of error-checking for bad values might alone
justify a custom type, depending on what you're doing.  A type with no
support beyond the required I/O functions could offer that.

But it's kinda hard to imagine a datatype in which there is no
meaningful way to define equality ...

            regards, tom lane