DB2 Union all vs并行运行

Question

We have separate Databases in DB2 for each customer but with same table structure in each of them. For a .Net Application I need to scan all the databases and show result for the matching entries to the user. I was wondering would it be faster to do a UNION ALL for all the databases or run each query in parallel and then combine them from my .Net Application.

Select EmpName, EmpSal, EmpDate
from A.Emptable
where EmpDate > '2015-01-01'
UNION ALL
Select EmpName, EmpSal, EmpDate
from B.Emptable
where EmpDate > '2015-01-01'
UNION ALL 
Select EmpName, EmpSal, EmpDate
from C.Emptable
where EmpDate > '2015-01-01'

VS.

Creating a .net Method GetEmpData to call each query and combine their results as:

var response = await Task.WhenAll(GetEmpData(A,'2015-01-01'),GetEmpData(B,'2015-01-01'),GetEmpData(C,'2015-01-01'));
var result = response[0].Concat(response[1]).Concat(response[2]).ToList();

Thanks.

Answer 1

If your federation is optimally configured for all aspects (especially pushdown and performance aspects), then I expect the UNION ALL to be more maintainable in the long term. As regards relative performance, you can measure, because so many factors influence that.