Question

I was looking through the sample LINQ queries provided with LINQPad taken from the C# 4.0 in a Nutshell book, and ran across something I have never used in LINQ to SQL… Compiled Queries.

Here is the exact exmaple:

// LINQ to SQL lets you precompile queries so that you pay the cost of translating
// the query from LINQ into SQL only once. In LINQPad the typed DataContext is
// called TypeDataContext, so we proceed as follows:

var cc = CompiledQuery.Compile ((TypedDataContext dc, decimal minPrice) =>    
    from c in Customers
    where c.Purchases.Any (p => p.Price > minPrice)
    select c
);

cc (this, 100).Dump ("Customers who spend more than $100");
cc (this, 1000).Dump ("Customers who spend more than $1000");

What does precompiling a LINQ to SQL query like this actually buy me? Would I get a performance boost from a query slightly more complex than this one? Is this even used in actual practice?

Answer 1

In a nutshell, precompiled querires buy you a performance gain when you need to run a single query multiple times.

Here’s some information on LINQ To SQL performance.

I’ve read in several places that
compiling your LINQ will help, but I
have never heard anyone say how
drastic the speed improvement can be.
For example, in one of my favorite
books (LINQ in Action) by Fabrice
Marguerie and others, he quotes on
page 296 a blog post by Rico Mariani
titled DLINQ (Linq to SQL Performance
(Part 1) as saying using a compiled
query offers nearly twice the
performanced of a non-compiled query,
and goes on to say that it brings the
performance to within 93% of using a
raw data reader. Well, suffice it to
say I never ran the test myself. I
could have lived with twice, but not
37 times.

So, from this, it seems that you
should always compile your LINQ to SQL
queries. Well, that’s not quite true.
What I’m recommending is that if you
have a reason to execute the same
query over and over you should
strongly consider compiling. If for
example, you are just making a LINQ to
SQL call once, there is no benefit
because you have to compile it anyway.
Call it ten times? Well, you will
have to decide for yourself.