Deferred Execution vs Immediate Execution of LINQ Query

In this blog, we will discuss about how deferred query execution and Immediate Query Execution works in LINQ, and what the difference between two are.

Deferred Execution: 

Deferred execution of LINQ query means it constructs the query/expression tree and it defers query execution until its value is requested. When value is required, it evaluate the execution tree in locally and then it send generated SQL to server. Deferred execution approach improves query execution performance by avoiding unnecessary database call.

Here is an example of Deferred LINQ query:

IQueryable<Order> orders = _dbContext.Orders.Where(x => x.OrderNumber == orderNumber);

foreach (Order item in orders)

      {

       new OrderData

       {

         OrderID = item.OrderID,

         OrderNumber = item.OrderNumber,

         OrderStatusCode = item.OrderStatu.StatusCode

       };

}

For above LINQ query, the SQL is not generated until the foreach statement executes.

Expression tree – expression tree is a data structure, which holds LINQ to SQL query, which will be sent to SQL server /database.

Immediate Execution: 

Immediate execution of LINQ means it enforces the LINQ query to execute and get the result immediately and there are many methods like ToList(),ToArray(), ToDictionary() executes the LINQ query immediately.

Here is an example of Immediate LINQ query: 

IList<Order> orders = _dbContext.Orders.Where(x => x.OrderNumber == orderNumber).ToList();

foreach (Order item in orders)

      {

       new OrderData

       {

         OrderID = item.OrderID,

         OrderNumber = item.OrderNumber,

         OrderStatusCode = item.OrderStatu.StatusCode

       };

}