Re: Anonymous code block with parameters

2014-09-16 9:10 GMT+02:00 Heikki Linnakangas <hlinnakangas@vmware.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