repro-pg-behavior-out-parameters

Repository holding code to reproduce unexpected behavior found with user defined functions with out parameters

I've found that in postgresql 9.3.10, when a user defined function has out parameters, it is invoked once per out parameter if invoked with the syntax:

SELECT (udf()).*

This syntax is desireable because it is the only way I've found so far to get the postgresql engine to return all of the out parameters together as a row, together with the parameters type information, instead of returning the out parameters together as the text representation of the composite type that they form together.

To demonstrate, take the function as follows:

CREATE FUNCTION demo(
  OUT param1 text,
  OUT param2 text,
  OUT param3 text
) AS $$
BEGIN
  param1 := 'foo';
  param2 := 'bar';
  param3 := 'baz';
END;
$$ LANGUAGE plpgsql

The query SELECT demo(); produces the result

testdb=# SELECT demo();
     demo      
---------------
 (foo,bar,baz)
(1 row)

Whereas the query SELECT (demo()).* produce the result

testdb=# SELECT (demo()).*;
 param1 | param2 | param3 
--------+--------+--------
 foo    | bar    | baz
(1 row)

I've yet to find another means to get postgresql to produce the result in such a form.

Unfortunately, I've found that the SELECT (udf()).* form executes the udf once per out parameter. This is undesireable for both performance reasons and unacceptable for functions that cause side effects. To demonstrate that this is happening I've created another function in reproduceBehavior.sql

Behavior occurs on: PostgreSQL 9.3.10 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, 64-bit

and

PostgreSQL 9.4.5 on x86_64-unknown-linux-gnu, compiled by gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3, 64-bit

Bug has been posted to pg-bugs on 12/5/15 and assigned #13799