Performance issue with ISNULL function

Hi all,



I have a case that cause performance issue as example below:



Table A:

Col1(PK)---Col2---IsActive



1------------C1-----1



2------------C2-----0

Table B:

Col1B(PK)---Col2B---Col3B(FK from table A)



1B-------------C1-----1



2B-------------C2-----NULL



When I use LEFT JOIN:



select B.* from table B left join table A on B.Col3B = A.Col1 where isnull(A.IsActive, 1) = 1



That means I need to get data in table B where A.IsActive = 1 or NULL.The above query have a problem with ISNULL function because with this function, we can not use the INDEX and when we have a BIG data, the performance is very BAD.



Is there any solution for this?



Thanks all