Should a subquery on a join use tables from an outer query in the where clause? - sql

Question

I need to add a subquery to a join, because one payment can have more than one allotment, so I only need to account for the first match (where rownum = 1).
However, I'm not sure if adding pmt from the outer query to the subquery on the allotment join is best.
Should I be doing this differently in the event of performance hits, etc.. ?
SELECT
pmt.payment_uid,
alt.allotment_uid,
FROM
payment pmt
/* HERE: is the reference to pmt.pay_key and pmt.client_id
incorrect in the below subquery? */
INNER JOIN allotment alc ON alt.allotment_uid = (
SELECT
allotment_uid
FROM
allotment
WHERE
pay_key = pmt.pay_key
AND
pay_code = 'xyz'
AND
deleted = 'N'
AND
client_id = pmt.client_id
AND
ROWNUM = 1
)
WHERE
AND
pmt.deleted = 'N'
AND
pmt.date_paid >= TO_DATE('2017-07-01')
AND
pmt.date_paid < TO_DATE('2017-10-01') + 1;

Answer 1

It's difficult to identify the performance issue in your query without seeing an explain plan output. You query does seem to do an additional SELECT on the allotment for every record from the main query.
Here is a version which doesn't use correlated sub query. Obviously I haven't been able to test it. It does a simple join in and then filters all records except one of the allotments. Hope this helps.
WITH v_payment
AS
(
SELECT
pmt.payment_uid,
alt.allotment_uid,
ROW_NUMBER () OVER(PARTITION BY allotment_id) r_num
FROM
payment pmt JOIN allotment alt
ON (pmt.pay_key = alt.pay_key AND
pmt.client_id = alt.client_id)
WHERE pmt.deleted = 'N' AND
pmt.date_paid >= TO_DATE('2017-07-01') AND
pmt.date_paid < TO_DATE('2017-10-01') + 1 AND
alt.pay_code = 'xyz' AND
alt.deleted = 'N'
)
SELECT payment_uid,
allotment_uid
FROM v_payment
WHERE r_num = 1;
Let's know how this performs!

Answer 2

You can phrase the query that way. I would be more likely to do:
SELECT . . .
FROM payment p INNER JOIN
(SELECT a.*,
ROW_NUMBER() OVER (PARTITION BY pay_key, client_id
ORDER BY allotment_uid
) as seqnum
FROM allotment a
WHERE pay_code = 'xyz' AND deleted = 'N'
) a
ON a.pay_key = p.pay_key AND a.client_id = p.client_id AND
seqnum = 1
WHERE p.deleted = 'N' AND
p.date_paid >= DATE '2017-07-01' AND
p.date_paid < (DATE '2017-10-01') + 1;