Most of the small or medium IT firms are using the Entity framework for the Data Access layer (DAL). If we write a complex linq to Entity queries performance will always be an issue. But with the Compiled Query Performance can be improved. This below definitions are from MSDN and more details can be found on the MSDN Link that is at the end of this post
When you have an application that executes structurally similar queries many times in the Entity Framework, you can frequently increase performance by compiling the query one time and executing it several times with different parameters. For example, an application might have to retrieve all the QuoteRevision for a particular quotelineStatus, the quotelinestatus is specified at runtime. LINQ to Entities supports using compiled queries for this purpose.
The compiled query class provides compilation and caching of queries for reuse. Conceptually, this class contains a CompiledQuery's Compile method with several overloads. Call the Compile method to create a new delegate to represent the compiled query. The Compile methods, provided with a object context(dbcontext) and can provide list of parameter values , return a delegate that produces some result (such as an IQueryable instance). The query compiles once during only the first execution. The merge options set for the query at the time of the compilation cannot be changed later. Once the query is compiled you can only supply parameters of primitive type but you cannot replace parts of the query that would change the generated SQL.
The LINQ to Entities query expression that the CompiledQuery's Compile method compiles is represented by one of the generic Func delegates, such as Func( Encapsulate a method that has diff parameters- ‘in our case object context, parameters and return type of T’, and a return value of the type specified).
Maxcimum, the query expression can encapsulate an ObjectContext parameter, a return parameter, and 16 query parameters. If more than 16 query parameters are required, you can create a structure whose properties represent query parameters. You can then use the properties on the structure in the query expression after you set the properties.
static Func<PricingToolEntities, Parameters, IQueryable<XX>> compiledQueryXXList =
CompiledQuery.Compile<PricingToolEntities, Parameters, IQueryable<XX>>(
(ctx, myparams) => (from x in ctx.XX
join y in ctx.YY
on x.Id equals y.xxId
join z in ctx.ZZ
on y.Id equals z.yyId
join a in ctx.AA
on z.AAId equals a.Id
where x.ExpiryDate >= myparams.param1&& x.IssueDate >= myparams.param2&& a.Name.ToUpper().Trim() == myparams.param3.ToUpper().Trim()
public DateTime param1;
public DateTime param2;
public string param3;
Parameters myParams = new Parameters();
IQueryable<XX> xxList = compiledQueryXXList.Invoke((Entities)dbContext, myParams);
For more information can be found in