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Re: returning multiple result sets from a stored procedure

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Darren Duncan <darren(at)darrenduncan(dot)net>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: returning multiple result sets from a stored procedure
Date: 2010-09-09 21:52:46
Message-ID: AANLkTimg6Dzy46zcuZpyw1XzTNjaQkG9THyXeer+9h4A@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
2010/9/9 Darren Duncan <darren(at)darrenduncan(dot)net>:
> Kevin Grittner wrote:
>>
>> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>>
>>> to my mind the main thing that would justify inventing a separate
>>> PROCEDURE facility is if procedures were to execute outside the
>>> transaction system, so that they could start and stop transactions
>>> for themselves.
>>
>>  That is the biggest distinction in my mind, too.  Supporting
>> multiple result sets just as if the queries were run as independent
>> client-side statements would also be very important.  I have seen
>> implementations which support, for a single stored procedure, OUT
>> parameters, a RETURN value, and multiple result sets -- all at the
>> same time, as separate things.  I haven't reviewed stored procedures
>> in the SQL standard since an early draft proposal years ago, so I
>> don't know what the current state of that is, but if PostgreSQL
>> approaches this, it'd be nice to implement as many of the above as
>> are not in conflict with requirements of the standard.
>
> If it was reasonable I would go further in splitting and have at least 4
> distinct kinds of routines, here listed in order of invocablility (each
> routine kind can invoke anything above it on the list but not anything below
> it):
>
> 1.  Expression-invoked pure functions that only have IN parameters and can
> not directly see the database or have any side-effects and are always in a
> transaction.  Most operators are of this kind.
>
> 2.  Statement-invoked routines that are pure like #1 but also have OUT/INOUT
> parameters instead of resulting in a value like a function.  The assignment
> operator is of this kind.
>
> 3.  Routines that *can* see and update the database but are otherwise like
> #2, and are always in a transaction.  The general case of a SELECT or DML or
> DDL are of this kind.
>
> 4.  Routines that can cross transaction boundaries or control transactions
> but are otherwise like #2 or #3.  Transaction control statements are of this
> kind.
>
> If I understand correctly, the existing Pg FUNCTION is essentially #3 and
> the proposed PROCEDURE is essentially #4.
>

Immutable functions are very near to #1. Actually PostgreSQL OUT
parameters are implemented as returned one composite value.

Regards

Pavel

> Maybe I just have to RTFM but I don't know if it is possible now to declare
> a Pg FUNCTION that it stays in the restrictions of #1 or #2.  But if not,
> then I think it would be valuable to do so, for assisting reliability and
> performance.
>
> -- Darren Duncan
>
>
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