在子查询中使用 FirstOrDefault

Question

We are having major performance issues with some of our LINQ-to-SQL queries.

We have a Case table, with a one-to-many relationship on CaseStatus (relationship table is CaseCaseStatus) Although, we want to keep track of all statusses on a case, only the last one really has any value.

We are building a query like follows:

IQueryable<Case> results = Repository.GetFullList(); // Returns a Queryable

if(filter.OnlyOpen){
   results = results
      .Where(c => c.CaseCaseStatusses.Any() 
               && c.CaseCaseStatusses.OrderByDescending(cs => cs.Timestamp).FirstOrDefault().Status.IsClosed != true);
}

results.Select(x => Dto(x)).ToList();

Basicly we want to know if the last status, has a property "IsClosed == true".

When I monitor my database through the SQL profiler, I can see that the database is being queried when we hit the Select statement, not before. (As intented) But while the query is being executed, I can see a select query for each case:

SELECT [cs0].[Id], [cs0].[CaseId], [cs0].[Note], [cs0].[ShowOnClientPage], [cs0].[StatusId], [cs0].[TimestampUTC], [cs.Status0].[IsClosed] 
FROM [CaseCaseStatusses] AS [cs0] 
INNER JOIN [CaseStatusses] AS [cs.Status0] ON [cs0].[StatusId] = [cs.Status0].[Id]
WHERE [cs0].[CaseId] = <CaseId>

Since we have 8000+ records in our Case Table, it does 8000 of above select statements... while we want 1 single query to be executed. Has this behavior changed in EFCore? I could swear I've done this before and it would output a single query (granted, it was a big unreadable query and not EFCore)

Is there another (better) way to perform these kind of queries?

Answer 1

Try this query, it should work with any EF version.

var results = 
   from r in results
   from c in r.CaseCaseStatusses
      .OrderByDescending(cs => cs.Timestamp)
      .Take(1)
   where c != null && c.Status.IsClosed != true
   select r;