Friday, February 12, 2010

How to implement paging with LINQ

Before going into how paging is implemented with LINQ, Let's discuss the need for implementing paging.

With large amounts of data, it is not a good practice to pull all records from database when you are showing a fraction of them in one page. It is always recommended to use data on demand approach. When you want to show first 20 records out of the search results then you must get the first 20 records from database and discard the rest. Similarly when you want to show next 20 records of the search results then you need to get the next 20 records from database and discard the rest. This is nothing but called paging.

LINQ has made the paging solution very simple as shown below example.

public List<Client>
GetAllClients(bool? isActive, int pageNumber, int pageSize, out int totalPages)
//Actual query which returns large data
var query = dataContext.Clients.Where(p => isActive == null || p.IsActive == isActive);

//Calculating total number of pages by taking ceiling number of the fractional value
totalPages = (int)Math.Ceiling((decimal)query.Count() / (decimal)pageSize);

//Paging logic goes here
return query.Skip((pageNumber - 1)*pageSize).Take(pageSize).ToList();

The parameters which play major role in paging are page number and page size. The page number is to identify the page of which the records to be returned. And the page size to identify the number of records to be returned. And there is another out parameter totalPages which is used to hold the total number of pages available within the data returned. This is needed to show the number of pages to the user and also useful in the logic which enables/disables page navigation.
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Kiran said...


I just have a question here concerning performance. In order to get the total number of records, you need to pull all the records from backend, right? In case of a very large table this can be a lot of data to retrieve and just throw away (for example if you have 10 000 records and just want to display 25 of them)

Kiran said...

The code query.Count() will not return the data but the total number of records satisfied the where condition. In other words the code is converted to sql as "select count(*) from table_name where where_condition". It returns only the number of the records those satisfied the condition rather than the data. I don't think this is inefficient.