What is equivalent of Firebird SQL 'for do' and 'select into' for postgresql - postgresql

Question

In a Firebird SQL stored procedure I use a 'select into' in a 'for do' loop and I don't find the equivalent for pg function.
for select purchase.quantity, purchase.purchasevalue, purchase.purchased, purchase.id from purchase
join cellarbook cb on purchase.fk_cellarbook_id = cb.id
join bottle bot on cb.fk_bottle_id = bot.id
where bot.id = :bottleid
order by purchase.purchased ASC
into :purquantity, :purvalue, :purdate, :purid
do
begin
/* calculate quantity on hand at point of purchase
here come some more 'select' and calculations and
then and 'update' */
select sum(psum.quantity) as purquantitysum from purchase
join cellarbook cb on psum.fk_cellarbook_id = cb.id
join bottle bot on cb.fk_bottle_id = bot.id
where bot.id = bottleid and psum.purchased <= pur.purchased and psum.id <> pur.id
into :purquantitysum
end
I think it is a 'for in loop' but I am hung up on what the equivalent for the 'select into' is.

Answer 1

You need to use a record variable for this:
declare
r record;
begin
for r in
select col_1, col_2 from some_table;
loop
select sum(x)
from other_table
where id = r.col_1;
end loop;
end;
More examples are in the manual:
http://www.postgresql.org/docs/current/static/plpgsql-control-structures.html#PLPGSQL-RECORDS-ITERATING
When you run update or select statements inside a loop is usually code-smell ("row-by-row processing"). In most of the cases it is much more efficient to do a bulk processing of everything in a single statement.

Answer 2

I'd use a cursor. There are several variations on the same theme available. I normally use this:
declare mycursor cursor for select a, b from c;
declare d, e bigint;
begin
loop
fetch from mycursor into d, e
exit when not found;
-- do your thing here
end loop;
close mycursor;
-- maybe do some other stuff
end;