Re: Relax requirement for INTO with SELECT in pl/pgsql - Mailing list pgsql-hackers

2016-04-10 18:49 GMT+02:00 David G. Johnston <david.g.johnston@gmail.com>:

On Sun, Apr 10, 2016 at 9:16 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:

2016-04-10 17:49 GMT+02:00 David G. Johnston <david.g.johnston@gmail.com>:

On Sun, Apr 10, 2016 at 1:13 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:

Hi

2016-03-21 22:13 GMT+01:00 Pavel Stehule <pavel.stehule@gmail.com>:

Hi

2016-03-21 21:24 GMT+01:00 Merlin Moncure <mmoncure@gmail.com>:

Patch is trivial (see below), discussion is not :-).

I see no useful reason to require INTO when returning data with
SELECT.  However, requiring queries to indicate not needing data via
PERFORM causes some annoyances:

*) converting routines back and forth between pl/pgsql and pl/sql
requires needless busywork and tends to cause errors to be thrown at
runtime

*) as much as possible, (keywords begin/end remain a problem),
pl/pgsql should be a superset of sql

*) it's much more likely to be burned by accidentally forgetting to
swap in PERFORM than to accidentally leave in a statement with no
actionable target.  Even if you did so in the latter case, it stands
to reason you'd accidentally leave in the target variable, too.

*) the PERFORM requirement hails from the days when only statements
starting with SELECT return data.  There is no PERFORM equivalent for
WITH/INSERT/DELETE/UPDATE and there are real world scenarios where you
might have a RETURNING clause that does something but not necessarily
want to place the result in a variable (for example passing to
volatile function).  Take a look at the errhint() clause below -- we
don't even have a suggestion in that case.

This has come up before, and there was a fair amount of sympathy for
this argument albeit with some dissent -- notably Pavel.  I'd like to
get a hearing on the issue -- thanks.  If we decide to move forward,
this would effectively deprecate PERFORM and the documentation will be
suitably modified as well.

here is another argument why this idea is not good.

http://stackoverflow.com/questions/36509511/error-query-has-no-destination-for-result-data-when-writing-pl-pgsql-function

Now, when people coming from T-SQL world use some T-SQL constructs, then usually the code should not work with the error "query has not destination for data ... "

When PLpgSQL will be more tolerant, then their code will be executed without any error, but will not work.

I would be inclined to require that DML returning tuples requires INTO while a SELECT does not.  Adding RETURNING is a deliberate user action that we can and probably should be conservative for.  Writing SELECT is default user behavior and is quite often used only for its side-effects.  Since SQL proper doesn't offer a means to distinguish between the two modes adding that distinction to pl/pgSQL, while useful, doesn't seem like something that has to be forced upon the user.

It doesn't help - SELECT is most often used construct.

We can be less strict for SELECT expr, but SELECT FROM should not be allowed without INTO clause.

​SELECT perform_this_action_for_every_user(user_id) FROM usertable;

I still only care about side-effects.

The rule remains (becomes?) simple:  Use INTO if you need to capture the SQL value into a pl/pgSQL variable - otherwise don't.  WRT your prior post I'd tell the user they are doing something really unusual if they write INSERT RETURNING without INTO - which I have no problem doing.

This is the problem. Any other databases doesn't allow it - or it has pretty different semantic (in T-SQL)

I am skeptical if benefit is higher than costs.

 

We don't need to force the user to tell us they intentionally omitted the INTO clause.  The omission itself is sufficient.  Using select without a target pl/pgSQL variable is a valid and probably quite common construct and hindering it because it might make debugging a function a bit harder (wrong data instead of an error) doesn't seem worthwhile.  You are making accommodations for exceptional situations.  I'm not convinced that it will be significantly harder to spot a missing INTO in a world where one is allowed to write such a statement without PERFORM.  Yes, it will not be as convenient.  Its a usability trade-off.

​There is value in having the non-procedural aspects of pl/pgSQL be as close to pure SQL as possible.​

It is not valid (semantically)  - you cannot throw result in pure SQL