How do you use SELECT...FOR UPDATE in a function? - postgresql

Question

Going off of this example, I am trying to lock someone's credit card account (row), check to see if they have enough money, and if they do, use it to pay. I need to lock it to prevent a condition where they have enough credit, but then it is used on another transaction and my program is left thinking it still does have enough credits.
At the terminal level I can accomplish this like so: I can open two psql sessions in two terminals and I can issue a SELECT * FROM credit_card WHERE credit_card_number = 1234 FOR UPDATE; command in one and then a SELECT * FROM credit_card FOR UDPATE in the other (or something else like UPDATE credit_card SET credits = credits -99 WHERE credit_card_number = 1234), and I can see that the former call blocks the latter call. However, when I do the same thing but in a function like so
CREATE OR REPLACE FUNCTION foo (p_credit_card_number BIGINT)
RETURNS VOID
AS $$
BEGIN
SELECT * FROM credit_card WHERE credit_card_number = p_credit_card_number FOR UPDATE
...
END
$$
LANGUAGE 'plpgsql';
I get the typical query has no destination for result data error (eg. see here).
Question: How can I lock a specific row, or number of rows, while using SELECT ...FOR UPDATE inside a Function while avoiding the above mentioned error?? If not possible, or not advisable, how else would I do this inside a Function?

Answer 1

The error you are getting has nothing to do whit the "for update" clause. It works exactly the same in a stored procedure of not. The error you has to to do this a select statement inside a procedure (or function or do block). When you select in a procedure you must tell the procedure what to do with the selected variable(s).
This you do with the Select <column_list> into <variable_list> ...
So assuming you actually need the data then something like the following:
create or replace function foo (p_credit_card_number bigint)
returns void
language 'plpgsql'
as $$
v_credit_card credit_card%type;
begin
select *
into v_credit_card
from credit_card
where credit_card_number = p_credit_card_number
for update;
...
update credit_card
set ...
where credit_card = v_credit_card ;
end;
$$;
A more common usage though is to create a cursor and update using 'where current of'
create or replace function foo (p_credit_card_number bigint)
returns void
language 'plpgsql'
as $$
c_credit_cards cursor for
select *
from credit_card
where credit_card_number = p_credit_card_number
for update;
v_credit_card credit_card%type;
begin
for v_cc in c_credit_cards
loop
... additional processing ...
update credit_card
set ...
where current of c_credit_cards;
end loop;
...
end;
But note that when the cursor is opened ALL rows in it are locked.