Re: PL/pgSQL PERFORM with CTE

On Wed, Aug 28, 2013 at 2:59 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Aug 27, 2013 at 6:10 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>> what is magical?
>>
>> Stored procedures - we talk about this technology was a originally simple
>> script moved from client side to server side.
>>
>> so if I write on client side
>>
>> BEGIN;
>> SELECT 1,2;
>> SELECT 2;
>> SELECT 3,4;
>> END;
>>
>> then I expect results
>>
>> 1,2
>> 2
>> 3,4
>
> The biggest problem with this idea is that people will do it by
> accident with unacceptable frequency. During the decade or so I
> worked as a web programmer, I made this mistake a number of times, and
> judging by the comments on this thread, Josh Berkus has made it with
> some regularity as well. If experienced PostgreSQL hackers who know
> the system inside and out make such mistakes with some regularity, I
> think we can anticipate that novices will make them even more often.
>
> And, TBH, as others have said here, I find the requirement to use
> PERFORM rather than SELECT rather ridiculous. The clash with CTEs has
> been there since we added CTEs, and I've hit it more than once. Yeah,
> you can work around it, but it's annoying. And why annoy people? So
> +1 from me for de-requiring the use of PERFORM (though I think we
> should definitely continue to accept that syntax, for backward
> compatibility).
>
> At the end of the day, procedural languages in PostgreSQL are
> pluggable. So if we someday have the ability to return extra result
> sets on the fly, and if Pavel doesn't like the syntax we choose to use
> in PL/pgsql, he can (and, given previous history, very possibly will!)
> publish his own PL with different syntax. But I'm with the crowd that
> says that's not the right decision for PL/pgsql.
>
> Also, even if we did adopt Pavel's proposed meaning for "SELECT 1,2",
> we still have a problem to solve, which is what the user should write
> when they want to run a query and ignore the results. The PERFORM
> solution was adequate at a time when all select queries started with
> SELECT, but now they can start with WITH or VALUES or TABLE as well,
> and while VALUES and TABLE may be ignorable, WITH certainly isn't.
> Requiring people to use silly workarounds like selecting into an
> otherwise-pointless dummy variable is not cool. If we reserve the
> undecorated-SELECT syntax to mean something else, then we've got to
> come up with some other way of solving David's original problem, and I
> don't think there are going to be many elegant options.
>
> Finally, I'd like to note that it's been longstanding frustration of
> mine that the PERFORM->SELECT transformation is leaky. For example,
> consider:
>
> rhaas=# do $$begin perform amazingly_well(); end;$$;
> ERROR: function amazingly_well() does not exist
> LINE 1: SELECT amazingly_well()
> ^
> HINT: No function matches the given name and argument types. You
> might need to add explicit type casts.
> QUERY: SELECT amazingly_well()
> CONTEXT: PL/pgSQL function inline_code_block line 1 at PERFORM
>
> Hmm, the user might say. I didn't type the word SELECT anywhere, yet
> it shows up in the error message. How confusing! With a big enough
> hammer we could perhaps paper over this problem a bit more thoroughly,
> but since I've never liked the syntax to begin with, I advance this as
> another argument for killing it.

Right. Another pain point for me is that I frequently have to
'up-convert' functions from sql to pgsql (and sometimes the other way
too). The perform requirement turns that into a headache. It looks
like we are mostly ok on Oracle compatibility too.

I'm a fan of David's 'YIELD' syntax concept as a line of analysis for
'mid procedure set returning' when we get there.

merlin