Hi
I have a view (MyView) which is

SELECT * FROM DB1.dbo.tableA
UNION 
SELECT * FROM DB2.dbo.TableB

I have SQL statement

SELECT myView.* FROM MyView
INNER JOIN DB2.dbo.TableZ ON myView.Key = TableZ.Key

It is working fine, returned result in 2 secs

However, if I amend it as below, it will take more than a minute to execute.

SELECT myView.*, TableZ.FieldA FROM MyView
INNER JOIN DB2.dbo.TableZ ON myView.Key = TableZ.Key

After some trial and error, found that, if I remark the edit my view without UNION (select only from TableB), the performance is 2 secs as well. Why?

Beside, I tried to add an index which contains Key + FieldA, but not helping at all.

Please advice, why remove the UNION can be helpful? How to solve this matter with "right" way?

Thank you