Linq to entity adding Where() clause breaks query - linq-to-sql

Question

Using Microsoft SQL Entity Framework I've got a query where sometimes I have a filter condition and sometimes I don't, so I tried to do what I've shown below. If the condition is not null then instead of doing the query as expected it queries everything from the Org_Hierarchy table, and then queries everything from the Workers table, and then dies as that takes too long:
void SomeMethod(Func<PRT, bool> whereClause) {
IQueryable<PRT> query;
if (whereClause != null) {
query = PRT.Where(whereClause).AsQueryable();
} else {
query = PRT.AsQueryable();
}
var data = from prt in query
// LEFT OUTER JOIN Worker a ON prt.assigned_to = a.WWID
join a_join in Worker on prt.assigned_to equals a_join.WWID into a_grp
from a in a_grp.DefaultIfEmpty()
// LEFT OUTER JOIN Worker c ON prt.closed_by = c.WWID
join c_join in Worker on prt.closed_by equals c_join.WWID into c_grp
from c in c_grp.DefaultIfEmpty()
// LEFT OUTER JOIN Worker r ON prt.requestor = r.WWID
join r_join in Worker on prt.requestor equals r_join.WWID into r_grp
from r in r_grp.DefaultIfEmpty()
// LEFT OUTER JOIN Org_Hierarchy o ON prt.org3 = o.OrganizationHierarchyUnitCd AND o.OrganizationHierarchyUnitTreeLevelNbr = 3 AND o.Active = true
join o in Org_Hierarchy on prt.org3 equals o.OrganizationHierarchyUnitCd
select new PrtInput {
If I change the query and put something direct in there, just for testing, like where prt.id == Guid.NewGuid() right above the last line shown then the query returns in one second. What's the trick to be able to dynamically add a where clause to the query?
The above code is from LinqPAD which is why the normal "context" stuff is all missing.

Answer 1

I'm not sure , but i think you should use something like this :
Expression<Func<PRT ,bool>> whereClause
Insted of:
Func<PRT ,bool> whereClause
When you using Func<> , first fetch data from db to memory then filter data in memory ,but if you use Epression<> filter send to sql and return result.
Also for the better performnce you can use AsNoTracking() like this:
if (whereClause != null) {
query = PRT.Where(whereClause).AsQueryable().AsNoTracking();
} else {
query = PRT.AsQueryable().AsNoTracking();
}
When you only want run query on yout database without any Insert ,update or delete on result , it better use AsNoTracking.
I hope this answers your question.