PostgreSQL questions - postgresql

Question

I am writing a function in PostgreSQL and wondering if I can do following:
I have an insert statement in every if loop. Can I pass values like this for formatdate1 and formatdate2?
I am also updating a table. Is it how we do it PostgreSQL?
CREATE OR REPLACE FUNCTION Check returns void AS $$
DECLARE
startDate=date;
formateDate1=date; formatdate2=date;newDate=date;
BEGIN
startDate:= SELECT to_date(lastdate::date, 'MM-DD-YYYY') FROM setup;
for i in 1..3 LOOP
IF i = 1 THEN
formateDate1 := select (startDate - INTERVAL '11 months');
formatdate2:= to_date(formatdate2::date,'YYYYMM');
insert into warehouse.memcnts1 (select distinct source,
formatdate2
as yearmo, to_date(formateDate1, 'MM-DD-YYYY')
where effdt <= formateDate1 and enddt >= formateDate1);
ELSIF i = 2 THEN -- this is todays date
--insert query here
insert into warehouse.memcnts1 (select distinct source,formatdate2 as yearmo, to_date(formateDate1, 'MM-DD-YYYY') where effdt <= formateDate1
and enddt >= formateDate1);
ELSIF i = 3 THEN
formateDate1 := select (startDate + INTERVAL '1 months');
newDate=formateDate1;
update dwset SET lastdate := newDate; -- wonder if this is right?
formatdate2:=startDate;
END IF;
END LOOP;
END
$$ language 'plpgsql';

Answer 1

In general any sort of insert, insert, update loop is going to run into problems if the loop gets large enough. I assume you want to do something like
FOR 1 .. n
LOOP
INSERT INTO foo VALUES (...);
INSERT INTO foo VALUES (...);
UPDATE BAR set ....;
END LOOP;
I make that assumption because there is no reason to do a loop and then run different logic for each one.
The problem with the above is that as n gets large (let's say, over a hundred or so) you can start to run into cache management issues which can suddenly start to cause a lot of random disk I/O activity and very long queries. There is nothing like wondering why a given function call runs through 50 iterations of a loop in, say, 100ms, but runs through 1000 iterations of a loop in 1800000 ms. (been there, done that).
In general you want to try to run your logic avoiding loops and instead perform set operations instead.
I hope this helps. It is really hard to see what you are asking.