Re: psql - add special variable to reflect the last query status

On Tue, Sep 12, 2017 at 3:12 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I think this is a bad plan. Right now, libpq sets no SQLSTATE for
>> internally generated errors; it is almost certain that there are
>> applications testing for an empty SQLSTATE to notice when they're
>> getting an error from libpq. EnterpriseDB had a support ticket quite
>> recently where this precise behavior was at issue. Changing it will
>> break stuff, so we shouldn't do it unless there's a really compelling
>> benefit. Universally returning PQ000 is not a sufficient improvement
>> over universally returning the empty string to justify the risk of
>> application breakage.
>
> I don't think I want to buy this argument, because the logical conclusion
> of it is that we can never fix libpq to offer proper SQLSTATEs for
> client-side errors. Admittedly, the fact that nobody's bothered to do so
> in ~15 years may indicate that nobody cares ... but I would think that
> at least it'd be useful to distinguish, say, ENOMEM from connection loss.
> Saying we can't do it for compatibility reasons doesn't sound great
> to me. Especially when you've not provided any hard evidence as to why
> the current lack-of-information is useful.

Well, if we provided a different SQLSTATE for each qualitatively
different type of libpq error, that might well be useful enough to
justify some risk of application breakage. But replacing a constant
string that we've had for ~15 years with a different constraint string
isn't doing anything about the lack-of-information problem you're
complaining about.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company