How to use variables in “EXECUTE format()” in plpgsql

I want to update a column in table stats with the specific column being a parameter, then return the updated value of that column [only has 1 row]:

CREATE FUNCTION grow(col varchar) RETURNS integer AS $$
DECLARE
tmp int;
BEGIN
    tmp := (EXECUTE format(
            'UPDATE stats SET %I = %I + 1
            RETURNING %I',
            col, col, col
            )
    );
    RETURN tmp;
END;

As a whole, I’m not even sure if this is best way to do what I want, any suggestion would be appreciated!

Answer

You can do that. Use the INTO keyword of the EXECUTE statement.

CREATE OR REPLACE FUNCTION grow(_col text, OUT tmp integer)
   LANGUAGE plpgsql AS
$func$
BEGIN

EXECUTE format(
 'UPDATE stats
  SET    %1$I = %1$I + 1
  RETURNING %1$I'
 , _col)
INTO tmp;

END
$func$;

Call:

SELECT grow('counter');

Using an OUT parameter to simplify overall.
format() syntax explained in the manual.

You could just run the UPDATE instead of a function call:

UPDATE stats SET counter = counter + 1 RETURNING counter;

There are not many scenarios where the function with dynamic SQL isn’t just needless complication.

Alternative design

If at all possible consider a different table layout: rows instead of columns (as suggested by @Ruslan). Allows any number of counters:

CREATE TABLE stats (
  tag text PRIMARY KEY
, counter int NOT NULL DEFAULT 0
);

Call:

UPDATE stats
SET    counter = counter + 1
WHERE  tag = 'counter1'
RETURNING counter;

Or maybe consider a dedicated SEQUENCE for counting …