Question

I have a very strange problem with a LINQ to Entities query with EF1.
I have a method with a simple query:

public DateTime GetLastSuccessfulRun(string job)
{
    var entities = GetEntities();
    var query = from jr in entities.JOBRUNS
                where jr.JOB_NAME == job && jr.JOB_INFO == "SUCCESS"
                orderby jr.JOB_END descending
                select jr.JOB_END;

    var result = query.ToList().FirstOrDefault();
    return result.HasValue ? result.Value : default(DateTime);
}

The method GetEntities returns an instance of a class that is derived from System.Data.Objects.ObjectContext and has automatically been created by the EF designer when I imported the schema of the database.
The query worked just fine for the last 15 or 16 months. And it still runs fine on our test system. In the live system however, there is a strange problem: Depending on the value of the parameter job, it returns the correct results or an empty result set, although there is data it should return.
Anyone ever had a strange case like that? Any ideas what could be the problem?

Some more info:
The database we query against is a Oracle 10g, we are using an enhanced version of the OracleEFProvider v0.2a.
The SQl statement that is returned by ToTraceString works just fine when executed directly via SQL Developer, even with the same parameter that is causing the problem in the LINQ query.
The following also returns the correct result:

entities.JOBRUNS.ToList().Where(x => x.JOB_NAME == job && x.JOB_INFO == "SUCCESS").Count();

The difference here is the call to ToList on the table before applying the where clause. This means two things:

1. The data is in the database and it is correct.
2. The problem seems to be the query including the where clause when executed by the EF Provider.

What really stuns me is, that this is a live system and the problem occurred without any changes to the database or the program. One call to that method returned the correct result and the next call five minutes later returned the wrong result. And since then, it only returns the wrong results.

Any hints, suggestions, ideas etc. are welcome, never mind, how far-fetched they seem! Please post them as answers, so I can vote on them, just for the fact for reading my lengthy question and bothering thinking about that strange problem… 😉

Answer 1

First of all remove ObjectContext caching. Object context internally uses UnitOfWork and IdentityMap patterns. This can have big impact on queries.