Tuning oracle subquery in select statement - sql

Question

I have a master table and a reference table as below.
WITH MAS as (
SELECT 10 as CUSTOMER_ID, 1 PROCESS_ID, 44 PROCESS_TYPE, 200 as AMOUNT FROM DUAL UNION ALL
SELECT 10 as CUSTOMER_ID, 1 PROCESS_ID, 44 PROCESS_TYPE, 250 as AMOUNT FROM DUAL UNION ALL
SELECT 10 as CUSTOMER_ID, 2 PROCESS_ID, 45 PROCESS_TYPE, 300 as AMOUNT FROM DUAL UNION ALL
SELECT 10 as CUSTOMER_ID, 2 PROCESS_ID, 45 PROCESS_TYPE, 350 as AMOUNT FROM DUAL
), REFTAB as (
SELECT 44 PROCESS_TYPE, 'A' GROUP_ID FROM DUAL UNION ALL
SELECT 44 PROCESS_TYPE, 'B' GROUP_ID FROM DUAL UNION ALL
SELECT 45 PROCESS_TYPE, 'C' GROUP_ID FROM DUAL UNION ALL
SELECT 45 PROCESS_TYPE, 'D' GROUP_ID FROM DUAL
) SELECT ...
My first select statement which works correctly is this one:
SELECT CUSTOMER_ID,
SUM(AMOUNT) as AMOUNT1,
SUM(CASE WHEN PROCESS_TYPE IN (SELECT PROCESS_TYPE FROM REFTAB WHERE GROUP_ID = 'A')
THEN AMOUNT ELSE NULL END) as AMOUNT2,
COUNT(CASE WHEN PROCESS_TYPE IN (SELECT PROCESS_TYPE FROM REFTAB WHERE GROUP_ID = 'D')
THEN 1 ELSE NULL END) as COUNT1
FROM MAS
GROUP BY CUSTOMER_ID
However, to address a performance issue, I changed it to this select statement:
SELECT CUSTOMER_ID,
SUM(AMOUNT) as AMOUNT1,
SUM(CASE WHEN GROUP_ID = 'A' THEN AMOUNT ELSE NULL END) as AMOUNT2,
COUNT(CASE WHEN GROUP_ID = 'D' THEN 1 ELSE NULL END) as COUNT1
FROM MAS A
LEFT JOIN REFTAB B ON A.PROCESS_TYPE = B.PROCESS_TYPE
GROUP BY CUSTOMER_ID
For the AMOUNT2 and COUNT1 columns, the values stay the same. But for AMOUNT1, the value is multiplied because of the join with the reference table.
I know I can add 1 more left join with an additional join condition on GROUP_ID. But that won't be any different from using a subquery.
Any idea how to make the query work with just 1 left join while not multiplying the AMOUNT1 value?

Answer 1

I know I can add 1 more left join with adding aditional GROUP_ID clause but it wont be different from subquery.
You'd be surprised. Having 2 left joins instead of subqueries in the SELECT gives the optimizer more ways of optimizing the query. I would still try it:
select m.customer_id,
sum(m.amount) as amount1,
sum(case when grpA.group_id is not null then m.amount end) as amount2,
count(grpD.group_id) as count1
from mas m
left join reftab grpA
on grpA.process_type = m.process_type
and grpA.group_id = 'A'
left join reftab grpD
on grpD.process_type = m.process_type
and grpD.group_id = 'D'
group by m.customer_id
You can also try this query, which uses the SUM() analytic function to calculate the amount1 value before the join to avoid the duplicate value problem:
select m.customer_id,
m.customer_sum as amount1,
sum(case when r.group_id = 'A' then m.amount end) as amount2,
count(case when r.group_id = 'D' then 'X' end) as count1
from (select customer_id,
process_type,
amount,
sum(amount) over (partition by customer_id) as customer_sum
from mas) m
left join reftab r
on r.process_type = m.process_type
group by m.customer_id,
m.customer_sum
You can test both options, and see which one performs better.

Answer 2

Starting off with your original query, simply replacing your IN queries with EXISTS statements should provide a significant boost. Also, be wary of summing NULLs, perhaps your ELSE statements should be 0?
SELECT CUSTOMER_ID,
SUM(AMOUNT) as AMOUNT1,
SUM(CASE WHEN EXISTS(SELECT 1 FROM REFTAB WHERE REFTAB.GROUP_ID = 'A' AND REFTAB.PROCESS_TYPE = MAS.PROCESS_TYPE)
THEN AMOUNT ELSE NULL END) as AMOUNT2,
COUNT(CASE WHEN EXISTS(SELECT 1 FROM REFTAB WHERE REFTAB.GROUP_ID = 'D' AND REFTAB.PROCESS_TYPE = MAS.PROCESS_TYPE)
THEN 1 ELSE NULL END) as COUNT1
FROM MAS
GROUP BY CUSTOMER_ID

Answer 3

The normal way is to aggregate the values before the group by. You can also use conditional aggregation, if the rest of the query is correct:
SELECT CUSTOMER_ID,
SUM(CASE WHEN seqnum = 1 THEN AMOUNT END) as AMOUNT1,
SUM(CASE WHEN GROUP_ID = 'A' THEN AMOUNT ELSE NULL END) as AMOUNT2,
COUNT(CASE WHEN GROUP_ID = 'D' THEN 1 ELSE NULL END) as COUNT1
FROM MAS A LEFT JOIN
(SELECT B.*, ROW_NUMBER() OVER (PARTITION BY PROCESS_TYPE ORDER BY PROCESS_TYPE) as seqnum
FROM REFTAB B
) B
ON A.PROCESS_TYPE = B.PROCESS_TYPE
GROUP BY CUSTOMER_ID;
This ignores the duplicates created by the joins.