SQL Server 2008 R2: Multiple UNION on different databases table's - sql

Question

I have two databases namely db1 and db2.
The database db1 contain 1 table namely test1 and database db2 contain two tables namely test2 and test3.
Here is the following table's with some demo records.
Database: db1
Table: test1
Create table test1
(
ID int,
hrs int,
dates date,
st_date date,
ed_date date
);
insert into test1 values(1,10,'2000-01-01','1900-01-01','2016-01-01');
insert into test1 values(2,20,'2000-02-01','1900-01-01','2016-01-01');
insert into test1 values(3,30,'2000-03-01','1900-01-01','2016-01-01');
insert into test1 values(4,40,'2000-04-01','1900-01-01','2016-01-01');
Database: db2
Table: test2
create table test2
(
ID int,
ID2 int
);
insert into test2 values(1,11);
insert into test2 values(2,22);
insert into test2 values(3,33);
insert into test2 values(4,44);
Database: db2
Table: test3
create table test3
(
ID int,
date date
);
insert into test3 values(1,'2000-01-01');
insert into test3 values(2,'2000-02-02');
insert into test3 values(3,'2000-03-03');
insert into test3 values(4,'2000-04-04');
Note: Now i am executing following query by union all three table's but getting performance issue. There are also test2,test3 tables are present in the db1
select nm,sum(x.avghrs)
from
(
select t1.nm, sum(hrs) as avghrs
from db2.test3 t3,
db2.test2 t2,
db1.test1 t1
where t1.id = t2.id
t3.id = t2.id
group by t1.nm
union
select t1.nm, sum(hrs) as avghrs
from db1.test3 t3,
db1.test2 t2,
db1.test1 t1
where t1.id = t2.id
t3.id = t2.id
group by t1.nm
union
select nm, 0 as avghrs
from test1
where dates between st_date and ed_date
) x
group by nm;
Please tell me if there is any need of modification?

Answer 1

I think the problem is related to JOINs between columns from tables residing in different databases. You can try the following:
1) mirror the tables in a single database (replicate schema and data)
2) apply the appropriate indexes (at least to contain ids used in JOINs)
3) Change your query to SELECT only from mirrored tables
Also, do you need to perform distinct for your unioned results? If not, UNION should be replaced with UNION ALL to avoid the implicit DISTINCT.

Answer 2

UNION ALL will perform better than UNION when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation.