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Re: enhanced error fields

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Peter Geoghegan <peter(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Stephen Frost <sfrost(at)snowman(dot)net>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Subject: Re: enhanced error fields
Date: 2013-01-05 16:55:03
Message-ID: CAFj8pRBzcEEfKhpqBsTiPSrEEnKRgg4aO2tVyBxgK5vYZu00Rg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Hello

2013/1/4 Robert Haas <robertmhaas(at)gmail(dot)com>:
> On Fri, Dec 28, 2012 at 1:21 PM, Peter Geoghegan <peter(at)2ndquadrant(dot)com> wrote:
>> Now, as to the question of whether we need to make sure that
>> everything is always fully qualified - I respectfully disagree with
>> Stephen, and maintain my position set out in the last round of
>> feedback [1], which is that we don't need to fully qualify everything,
>> because *for the purposes of error handling*, which is what I think we
>> should care about, these fields are sufficient:
>>
>> +   column_name text,
>> +   table_name text,
>> +   constraint_name text,
>> +   schema_name text,
>>
>> If you use the same constraint name everywhere, you might get the same
>> error message. The situations in which this scheme might fall down are
>> too implausible for me to want to bloat up all those ereport sites any
>> further (something that Stephen also complained about).
>>
>> I think that the major outstanding issues are concerning whether or
>> not I have the API here right. I make explicit guarantees as to the
>> availability of certain fields for certain errcodes (a small number of
>> "Class 23 - Integrity Constraint Violation" codes). No one has really
>> said anything about that, though I think it's important.
>>
>> I also continue to think that we shouldn't have "routine_name",
>> "routine_schema" and "trigger_name" fields - I think it's wrong-headed
>> to have an exception handler magically act on knowledge about where
>> the exception came from that has been baked into the exception - is
>> there any sort of precedent for this? Pavel disagrees here. Again, I
>> defer to others.
>
> I don't really agree with this.  To be honest, my biggest concern
> about this patch is that it will make it take longer to report an
> error.  At least in the cases where these additional fields are
> included, it will take CPU time to populate those fields, and maybe
> there will be some overhead even in the cases where they aren't even
> used (although I'd expect that to be too little to measure).  Now,
> maybe that doesn't matter in the case where the error is being
> reported back to the client, because the overhead of shipping packets
> across even a local socket likely dwarfs the overhead, but I think it
> matters a lot where you are running a big exception-catching loop.
> That is already painfully slow, and -1 from me on anything that makes
> it significantly slower.  I have a feeling this isn't the first time
> I'm ranting about this topic in relation to this patch, so apologies
> if this is old news.

We did these tests independently - Peter and me with same result - in
use cases developed for highlighting impact of this patch you can see
some slowdown - if I remember well - less than 3% - and it raised
exceptions really intensively - and It can be visible only from stored
procedures environment - if somebody use it from client side, then
impact is zero.

>
> But if we decide that there is no performance issue or that we don't
> care about the hit there, then I think Stephen and Pavel are right to
> want a large amount of very precise detail.  What's the use case for
> this feature?  Presumably, it's for people for whom parsing the error
> message is not a practical option, so either they textual error
> message doesn't identify the target object sufficiently precisely, and
> they want to make sure they know what it applies to; or else it's for
> people who want any error that applies to a table to be handled the
> same way (without worrying about exactly which error they have got).
> Imagine, for example, someone writing a framework that will be used as
> a basis for many different applications.  It might want to do
> something, like, I don't know, update the comment on a table every
> time an error involving that table occurs.  Clearly, precise
> identification of the table is a must.  In a particular application
> development environment, it's reasonable to rely on users to name
> things sensibly, but if you're shipping a tool that might be dropped
> into any arbitrary environment, that's significantly more dangerous.
>
> Similarly, for a function-related error, you'd need something like
> that looks like the output of pg_proc.oid::regprocedure, or individual
> fields with the various components of that output.  That sounds like
> routine_name et. al.

Probably we don't need all fields mentioned in ANSI SQL, because some
from these fields has no sense in pg, but routine name and trigger
name is really basic for some little bit more sophisticate work with
exception on PL level.

Regards

Pavel


>
> I'm not happy about the idea of shipping OIDs back to the client.
> OIDs are too user-visible as it is; we should try to make them less
> not moreso.
>
> --
> Robert Haas
> EnterpriseDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company


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