Re: Anonymous code block with parameters

On 09/16/2014 09:15 AM, Pavel Stehule wrote:
>
>
> 2014-09-16 9:10 GMT+02:00 Heikki Linnakangas <hlinnakangas(at)vmware(dot)com
> <mailto:hlinnakangas(at)vmware(dot)com>>:
>
> On 09/16/2014 09:38 AM, Kalyanov Dmitry wrote:
>
> I'd like to propose support for IN and OUT parameters in 'DO'
> blocks.
>
> Currently, anonymous code blocks (DO statements) can not
> receive or
> return parameters.
>
> I suggest:
>
> 1) Add a new clause to DO statement for specifying names, types,
> directions and values of parameters:
>
> DO <code> [LANGUAGE <lang>] [USING (<arguments>)]
>
> where <arguments> has the same syntax as in
> 'CREATE FUNCTION <name> (<arguments>)'.
>
> Example:
>
> do $$ begin z := x || y; end; $$
> language plpgsql
> using
> (
> x text = '1',
> in out y int4 = 123,
> out z text
> );
>
> 2) Values for IN and IN OUT parameters are specified using
> syntax for
> default values of function arguments.
>
> 3) If DO statement has at least one of OUT or IN OUT
> parameters then it
> returns one tuple containing values of OUT and IN OUT parameters.
>
> Do you think that this feature would be useful? I have a
> proof-of-concept patch in progress that I intend to publish soon.
>
>
> There are two features here. One is to allow arguments to be
> passed to DO statements. The other is to allow a DO statement to
> return a result. Let's discuss them separately.
>
> 1) Passing arguments to a DO block can be useful feature, because
> it allows you to pass parameters to the DO block without injecting
> them into the string, which helps to avoid SQL injection attacks.
>
> I don't like the syntax you propose though. It doesn't actually
> let you pass the parameters out-of-band, so I don't really see the
> point. I think this needs to work with PREPARE/EXECUTE, and the
> protocol-level prepare/execute mechanism. Ie. something like this:
>
> PREPARE mydoblock (text, int4) AS DO $$ ... $$
> EXECUTE mydoblock ('foo', 123);
>
> 2) Returning values from a DO block would also be handy. But I
> don't see why it should be restricted to OUT parameters. I'd
> suggest allowing a RETURNS clause, like in CREATE FUNCTION:
>
> DO $$ ... $$ LANGUAGE plpgsql RETURNS int4;
>
> or
>
> DO $$ ... $$ LANGUAGE plpgsql RETURNS TABLE (col1 text, col2 int4);
>
> - Heikki
>
>
> Why we don't introduce a temporary functions instead?

As I see it, the DO blocks _are_ temporary (or rather in-line)
functions, though quite restricted in not taking arguments and not
returning anything.

DO you have a better syntax for "temporary / in-line functions" ?

What I would like to to is to make DO blocks equal to any other data
source, so you could do

WITH mydoblock(col1, col2)(DO $$ ... $$ LANGUAGE plpgsql RETURNS TABLE
(col1 text, col2 int4))
SELECT * FROM mydoblock;

or

SELECT *
FROM (DO $$ ... $$ LANGUAGE plpgsql RETURNS TABLE (col1 text, col2
int4)) mydoblock;

and for the parameter-taking version

SELECT (DO $$ ... $$ LANGUAGE plpgsql USING (user) RETURNS
int4)(username) AS usernum
FROM users;

Cheers

--
Hannu Krosing
PostgreSQL Consultant
Performance, Scalability and High Availability
2ndQuadrant Nordic OÜ