Issue with the performance of the query - sql

Question

I have two tables tab1 and tab2 each are having two columns acc_num and prod_code. I need to update prod_code in tab2 from tab1. Below is the sample data in both the tables:
TAB1
acnum Prod
-------------------
1 A
2 B
2 C
3 X
3 X
Tab2
acnum Prod
-------------------
1 null
2 null
2 null
3 null
3 null
And for the 2nd table after update all the distinct codes should be concatenated. Below is the sample output.
Tab2
acnum Prod
-------------------
1 A
2 B|C
2 B|C
3 X
3 X
I am able to achieve this through PL/SQL, but it's taking ages to complete. (Actual tables are having millions of records). Below is the code I am using.
DECLARE
l_acnum dbms_sql.varchar2a;
l_prod dbms_sql.varchar2a;
l_prod2 VARCHAR2(10):= NULL;
l_count NUMBER := 0;
CURSOR cr_acnum
IS
SELECT DISTINCT(acnum) FROM tab1;
CURSOR cr_prod(l_acnum_dum IN VARCHAR2)
IS
SELECT prod FROM tab1 WHERE acnum = l_acnum_dum;
BEGIN
OPEN cr_acnum;
FETCH cr_acnum bulk collect INTO l_acnum;
CLOSE cr_acnum;
FOR i IN l_acnum.first .. l_acnum.last
LOOP
OPEN cr_prod(l_acnum(i));
FETCH cr_prod bulk collect INTO l_prod;
CLOSE cr_prod;
FOR m IN l_prod.first .. l_prod.last
LOOP
IF m <> 1 THEN
IF l_prod(m) = l_prod(m-1) THEN
l_prod2 := l_prod(m);
ELSE
l_prod2 := l_prod2||'|'||l_prod(m);
END IF;
ELSE
l_prod2 := l_prod(m);
END IF;
END LOOP;
UPDATE tab2 SET prod = l_prod2 WHERE acnum = l_acnum(i);
END LOOP;
END;
This pl/sql block is taking ages to complete. Is there anyway I can achieve the same through query rather than PL/SQL or may be by efficient PL/SQL. I tried BULK COLLECT also but of no use. Data is in Oracle DB. Thanks a lot for your time.

Answer 1

This will concatenate those values as long as they don't exceed a certain total length. You may also want to do a subquery to dedupe them if there are any dupes.
Update: here is with LISTAGG
update table2 set Prod = (
SELECT LISTAGG(t1.Prod, ', ') WITHIN GROUP (ORDER BY t1.Prod) "Prod"
FROM Table1 t1
where t1.acnum = table2.acnum)

Answer 2

Thanks everyone for your input. I achived the same using your inputs. I used 2 step solution:
Step 1) Create a lookup table for product and account no.
create table lkup_tbl
as SELECT acnum, LISTAGG(Prod, '|') WITHIN GROUP (ORDER BY Prod) product
FROM (select distinct acnum, Prod from tab1) tab
GROUP BY acnum;
Step 2) Now update all the tables by joining this lookup table.
update tab2 t1
set (t1.Prod) = (select product from lkup_tbl t2
where t2.acnum = t1.acnum
);