SQL Server Slow query using subqueries - sql

Question

I have a very slooowwwww query.
It selects customer records, with combined criteria, kinda like this:
A table has Customers, another table has CustomerCars, another table has CustomerMotorcycles.
A customer can have one or more cars. If the customer is a commercial business and any one of the customer's cars is a Ford, then we want to exclude that customer from our selection.
A customer can also have one or more motorcycles, and if the customer is a retail business and any one of its motorcycles is a Harley, then we want to exclude that customer.
So I have a statement like:
SELECT *
FROM CUST
WHERE
(CUST.CUSTTYPE = 'COMM'
AND CUST.CUSTID NOT IN (SELECT CUSTCARS.CUSTID
FROM CUSTCARS
WHERE CUSTCARS.CAR = 'FORD'))
OR
(CUST.CUSTTYPE = 'RETAIL'
AND CUST.CUSTID NOT IN (SELECT CUSTCYCLES.CUSTID
FROM CUSTCYCLES
WHERE CUSTCYCLES.CYCLE = 'HARLEY'))
This runs crazy slow.
This is currently being run as a bunch of separate queries that dump data into temporary tables, then several other queries are run to delete the records we don't want, but it's quite clumsy.
Any suggestions? Thanks for any help!

Answer 1

Try "left excluding joins", where we left join the data that matches the conditions we do not want, and then exclude the matching rows through the where clause:
SELECT
CUST.*
FROM CUST
LEFT JOIN CUSTCARS ON CUST.CUSTID = CUSTCARS.CUSTID
AND CUSTCARS.CAR = 'FORD'
AND CUST.CUSTTYPE = 'COMM'
LEFT JOIN CUSTCYCLES ON CUST.CUSTID = CUSTCYCLES.CUSTID
AND CUSTCYCLES.CYCLE = 'HARLEY'
AND CUST.CUSTTYPE = 'RETAIL'
WHERE CUSTCARS.CUSTID IS NULL
OR CUSTCYCLES.CUSTID IS NULL
;
Whilst I'm here, it might be the OR in your existing query that causes excessive slowness (maybe) so perhaps combining the 2 subqueries to one list would help:
SELECT
*
FROM CUST
WHERE CUSTID NOT IN (
SELECT
CUSTCARS.CUSTID
FROM CUSTCARS
INNER JOIN CUST ON CUSTCARS.CUSTID = CUST.CUSTID
WHERE CUSTCARS.CAR = 'FORD'
AND CUST.CUSTTYPE = 'COMM'
UNION ALL
SELECT
CUSTCYCLES.CUSTID
FROM CUSTCYCLES
INNER JOIN CUST ON CUSTCYCLES.CUSTID = CUST.CUSTID
WHERE CUSTCYCLES.CYCLE = 'HARLEY'
AND CUST.CUSTTYPE = 'RETAIL'
)
;

Answer 2

Two things come to mind. First, make sure there are indexes for CUSTCARS.CAR and CUSTCYCLES.CYCLE. Second, you might try NOT EXISTS instead of NOT IN.
SELECT * FROM CUST
WHERE
(CUST.CUSTTYPE = 'COMM'
AND NOT EXISTS(SELECT 1 FROM CUSTCARS WHERE CUSTCARS.CAR = 'FORD'
AND CUSTCARS.CUSTID=CUST.CUSTID))
OR
(CUST.CUSTTYPE = 'RETAIL'
AND NOT EXISTS(SELECT 1 FROM CUSTCYCLES WHERE CUSTCYCLES.CYCLE = 'HARLEY'
AND CUSTCYCLES.CUSTID=CUST.CUSTID))

Answer 3

I would suggest starting with not exists:
select c.*
from cust c
where not (c.custtype = 'COMM' and
exists (select 1
from custcars cc
where cc.custid =c.custid and cc.car = 'FORD'
)
) and
not (c.custtype = 'RETAIL' and
exists (select 1
from custcycles cc
where cc.custid = c.custid and cc.cycle = 'HARLEY'
)
) ;
Then, you want to be sure you have indexes on custcar(custid, car) and custcycles(custid, cycle).