Is there a way to make this query more efficient performance wise? - sql

Question

This query takes a long time to run on MS Sql 2008 DB with 70GB of data.
If i run the 2 where clauses seperately it takes a lot less time.
EDIT - I need to change the 'select *' to 'delete' afterwards, please keep it in mind when answering. thanks :)
select *
From computers
Where Name in
(
select T2.Name
from
(
select Name
from computers
group by Name
having COUNT(*) > 1
) T3
join computers T2 on T3.Name = T2.Name
left join policyassociations PA on T2.PK = PA.EntityId
where (T2.EncryptionStatus = 0 or T2.EncryptionStatus is NULL) and
(PA.EntityType <> 1 or PA.EntityType is NULL)
)
OR
ClientId in
(
select substring(ClientID,11,100)
from computers
)

Answer 1

Swapping IN for EXISTS will help.
Also, as per Gordon's answer: UNION can out-perform OR.
SELECT computers.*
FROM computers
LEFT
JOIN policyassociations
ON policyassociations.entityid = computers.pk
WHERE (
computers.encryptionstatus = 0
OR computers.encryptionstatus IS NULL
)
AND (
policyassociations.entitytype <> 1
OR policyassociations.entitytype IS NULL
)
AND EXISTS (
SELECT name
FROM (
SELECT name
FROM computers
GROUP
BY name
HAVING Count(*) > 1
) As duplicate_computers
WHERE name = computers.name
)
UNION
SELECT *
FROM computers As c
WHERE EXISTS (
SELECT SubString(clientid, 11, 100)
FROM computers
WHERE SubString(clientid, 11, 100) = c.clientid
)
You've now updated your question asking to make this a delete.
Well the good news is that instead of the "OR" you just make two DELETE statements:
DELETE
FROM computers
LEFT
JOIN policyassociations
ON policyassociations.entityid = computers.pk
WHERE (
computers.encryptionstatus = 0
OR computers.encryptionstatus IS NULL
)
AND (
policyassociations.entitytype <> 1
OR policyassociations.entitytype IS NULL
)
AND EXISTS (
SELECT name
FROM (
SELECT name
FROM computers
GROUP
BY name
HAVING Count(*) > 1
) As duplicate_computers
WHERE name = computers.name
)
;
DELETE
FROM computers As c
WHERE EXISTS (
SELECT SubString(clientid, 11, 100)
FROM computers
WHERE SubString(clientid, 11, 100) = c.clientid
)
;

Answer 2

Some things I would look at are
1. are indexes in place?
2. 'IN' will slow your query, try replacing it with joins,
3. you should use column name, I guess 'Name' in this case, while using count(*),
4. try selecting required data only, by selecting particular columns.
Hope this helps!

Answer 3

or can be poorly optimized sometimes. In this case, you can just split the query into two subqueries, and combine them using union:
select *
From computers
Where Name in
(
select T2.Name
from
(
select Name
from computers
group by Name
having COUNT(*) > 1
) T3
join computers T2 on T3.Name = T2.Name
left join policyassociations PA on T2.PK = PA.EntityId
where (T2.EncryptionStatus = 0 or T2.EncryptionStatus is NULL) and
(PA.EntityType <> 1 or PA.EntityType is NULL)
)
UNION
select *
From computers
WHERE ClientId in
(
select substring(ClientID,11,100)
from computers
);
You might also be able to improve performance by replacing the subqueries with explicit joins. However, this seems like the shortest route to better performance.
EDIT:
I think the version with join's is:
select c.*
From computers c left outer join
(select c.Name
from (select c.*, count(*) over (partition by Name) as cnt
from computers c
) c left join
policyassociations PA
on T2.PK = PA.EntityId and PA.EntityType <> 1
where (c.EncryptionStatus = 0 or c.EncryptionStatus is NULL) and
c.cnt > 1
) cpa
on c.Name = cpa.Name left outer join
(select substring(ClientID, 11, 100) as name
from computers
) csub
on c.Name = csub.name
Where cpa.Name is not null or csub.Name is not null;