Find out if a parameter/return field is NULLABLE using Describe

It doesn't seem to be possible to find out if a parameter or result
field is nullable using the Describe protocol.

For example if I have this table and statement:

create table employees (
id serial not null primary key,
name text not null,
email text <-- this field
);

insert into employees (name, email) values ($1, $2);

Then the only information I get back about $2 is that it's a TEXT (OID
= 25), when clearly $1 and $2 are different in that one is nullable
and the other isn't.

Similarly,

select name, email from employees;

This time I get more information about the types, including a modifier
and length, but I can't see whether that lets me find out that one
could be NULL and the other can never be.

So is this possible?

Rich.

--
Richard Jones, CTO Merjis Ltd.
Merjis - web marketing and technology - http://merjis.com
Team Notepad - intranets and extranets for business - http://team-notepad.com

Richard Jones <rich(at)annexia(dot)org> writes:
> It doesn't seem to be possible to find out if a parameter or result
> field is nullable using the Describe protocol.

This isn't something that the backend tracks at all, so changing the
wire protocol would be the least of your problems in making it happen...

regards, tom lane

On Mon, 30 Jan 2006, Tom Lane wrote:

> Richard Jones <rich(at)annexia(dot)org> writes:
>> It doesn't seem to be possible to find out if a parameter or result
>> field is nullable using the Describe protocol.
>
> This isn't something that the backend tracks at all, so changing the
> wire protocol would be the least of your problems in making it happen...
>

Well, the protocol does attempt to return the resulting columns' source
table and column number which may then be used to determine the
nullability of the field. This won't work in all cases, but for the
simple select example given, it will.

Kris Jurka