Re: enhanced error fields - Mailing list pgsql-hackers

On 29 December 2012 05:04, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> again - it is reason why I propose ROUTINE_NAME and TRIGGER_NAME - it
> can be useful for some use cases, where developer should to handle
> exception when they coming from known functions/triggers and he would
> to raise exception, when it was raised elsewhere. Is possible working
> with CONTEXT, but it needs little bit more work and situation is very
> similar to fields COLUMN_NAME, where I can parse message and I can get
> this information. But then it depends on message format.

That seems very thin. Again, I have to point out that a precedent for
this doesn't really appear to exist in any major programming language,
even though ISTM it would be just as useful in a wide variety of
programming environments as it would be within Postgres. As I've said,
the DB2 GET DIAGNOSTIC stuff isn't anything like what you've proposed.

>> So I'm with Peter G on this: the existing CONTEXT mechanism is good
>> enough, we do not need to split out selected sub-parts of that as
>> separate error fields.  Moreover, doing so would provide only an utterly
>> arbitrary subset of the context stack: who's to say whether it would be
>> more important to report the most closely nested function, or the
>> outermost one?
>
> I don't agree with this argument - you can say too "COLUMN_NAME,
> TABLE_NAME" is not necessary, because MESSAGE is good enough. I don't
> see any difference - and I would to use these variables for error
> handling (not for logging) without dependency on current format of
> MESSAGE or CONTEXT.

In my judgement, COLUMN_NAME and TABLE_NAME can be used without having
an unreasonable degree of coupling between client and server-side
code. They are at least easily understood, and not at all astonishing,
unlike ROUTINE_NAME. That said, CONSTRAINT_NAME clearly is by far the
most compelling new field, and I actually wouldn't mind losing the
other ones too much. I might have suggested losing COLUMN_NAME and
TABLE_NAME myself if we could reliably get something like a constraint
name for NOT NULL violations.

> Second question - what should be context is important. I am thinking
> so standard and implementation in other databases is relative clean -
> it is name of custom routine, that doesn't handle exception. Moving to
> pg - it is name of routine on the top on error callback stack, because
> builtin functions doesn't set it - I don't need on top of CONTEX -
> div_int or other name of builtin function - but I need there function
> foo(b): a := 10/b.

I don't think the fact that built-in functions don't set ROUTINE_NAME
supports your position. In fact, it seems pretty broken to me that one
pl handler sets the value, while others may not. Furthermore, the
distinction between built-in and not built-in is fairly small within
Postgres - who is to say that even if a person thinks the proposed
semantics are useful, they'll continue to when they find out that
ROUTINE_NAME isn't set to the name of their C function?

> other point - theoretically I can get ROUTINE_NAME from CONTEXT, but I
> cannot get TRIGGER_NAME - this information is lost.

I don't think there should be a TRIGGER_NAME either - I think that we
should make interfaces easy to use correctly, and hard to use
incorrectly, and a mechanised response to a TRIGGER_NAME seems
incorrect. If you think that there should be a trigger name within
CONTEXT, there might be a case to be made for that, but I would prefer
to have that reviewed separately.

-- 
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services