Linq To Entities - Any VS First VS Exists

Question

I am using Entity Framework and I need to check if a product with name = \"xyz\" exists ...

I think I can use Any(), Exists() or First().

Which one is the b

Answer 1

Ok, I decided to try this out myself. Mind you, I'm using the OracleManagedDataAccess provider with the OracleEntityFramework, but I'm guessing it produces compliant SQL.

I found that First() was faster than Any() for a simple predicate. I'll show the two queries in EF and the SQL that was generated. Mind you, this is a simplified example, but the question was asking whether any, exists or first was faster for a simple predicate.

 var any = db.Employees.Any(x => x.LAST_NAME.Equals("Davenski"));

So what does this resolve to in the database?

SELECT 
CASE WHEN ( EXISTS (SELECT 
1 AS "C1"
FROM "MYSCHEMA"."EMPLOYEES" "Extent1"
WHERE ('Davenski' = "Extent1"."LAST_NAME")
 )) THEN 1 ELSE 0 END AS "C1"
FROM  ( SELECT 1 FROM DUAL ) "SingleRowTable1"

It's creating a CASE statement. As we know, ANY is merely syntatic sugar. It resolves to an EXISTS query at the database level. This happens if you use ANY at the database level as well. But this doesn't seem to be the most optimized SQL for this query. In the above example, the EF construct Any() isn't needed here and merely complicates the query.

 var first = db.Employees.Where(x => x.LAST_NAME.Equals("Davenski")).Select(x=>x.ID).First();

This resolves to in the database as:

SELECT 
"Extent1"."ID" AS "ID"
FROM "MYSCHEMA"."EMPLOYEES" "Extent1"
WHERE ('Davenski' = "Extent1"."LAST_NAME") AND (ROWNUM <= (1) )

Now this looks like a more optimized query than the initial query. Why? It's not using a CASE ... THEN statement.

I ran these trivial examples several times, and in ALMOST every case, (no pun intended), the First() was faster.

In addition, I ran a raw SQL query, thinking this would be faster:

var sql = db.Database.SqlQuery("SELECT ID FROM MYSCHEMA.EMPLOYEES WHERE LAST_NAME = 'Davenski' AND ROWNUM <= (1)").First();

The performance was actually the slowest, but similar to the Any EF construct.

Reflections:

1. EF Any doesn't exactly map to how you might use Any in the database. I could have written a more optimized query in Oracle with ANY than what EF produced without the CASE THEN statement.
2. ALWAYS check your generated SQL in a log file or in the debug output window.
3. If you're going to use ANY, remember it's syntactic sugar for EXISTS. Oracle also uses SOME, which is the same as ANY. You're generally going to use it in the predicate as a replacement for IN. In this case it generates a series of ORs in your WHERE clause. The real power of ANY or EXISTS is when you're using Subqueries and are merely testing for the EXISTENCE of related data.

Here's an example where ANY really makes sense. I'm testing for the EXISTENCE of related data. I don't want to get all of the records from the related table. Here I want to know if there are Surveys that have Comments.

var b = db.Survey.Where(x => x.Comments.Any()).ToList();

This is the generated SQL:

SELECT 
"Extent1"."SURVEY_ID" AS "SURVEY_ID", 
"Extent1"."SURVEY_DATE" AS "SURVEY_DATE"
FROM "MYSCHEMA"."SURVEY" "Extent1"
WHERE ( EXISTS (SELECT 
1 AS "C1"
FROM "MYSCHEMA"."COMMENTS" "Extent2"
WHERE ("Extent1"."SURVEY_ID" = "Extent2"."SURVEY_ID")
 ))

This is optimized SQL! I believe the EF does a wonderful job generating SQL. But you have to understand how the EF constructs map to DB constructs else you can create some nasty queries.

And probably the best way to get a count of related data is to do an explicit Load with a Collection Query count. This is far better than the examples provided in prior posts. In this case you're not loading related entities, you're just obtaining a count. Here I'm just trying to find out how many Comments I have for a particular Survey.

  var d = db.Survey.Find(1);

  var  e = db.Entry(d).Collection(f => f.Comments)
                               .Query()
                               .Count();