House3272 House3272 - 1 month ago 6
SQL Question

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) AS
$func$
BEGIN

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

END
$func$ LANGUAGE plpgsql;

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 ...

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