Paging is the way to browse through a list of objects or rows of data one page at a time. This can be handy when you have thousands of rows / objects matching search criteria but you want to enlist only a small number at once. With the paging functionality build into entity collection classes and typed list/typed view classes, you can tell the generated code which page to retrieve, instead of getting all the results at once.
Queries using Linq to LLBLGen or QuerySpec are using paging constructs of those systems and are described in their own sections: Paging in Linq and Paging in QuerySpec. This section describes the various options available to you when fetching elements using the Low-level API.
Paging in general happens on the server, however the LLBLGen Pro runtime framework switches to client-side paging at the datareader level if the paging query can't be generated without causing errors on the server. This happens if a DISTINCT violating construct is present in the SQL query, e.g. a type that's not to be used with DISTINCT or an order by on a field that's not in the projection. The datareader-level paging is efficient (it discards any rows before the page to read and stops reading data once the page to read is fully read) however can be slower than a server-side paging query.
To determine whether the framework switched to client-side paging / distinct filtering / row limiting, it will append to the SQL query output to the ORMQueryExecution tracer a string which signals this: "Requires client side paging", "Requires client side distinct filtering" and "Requires client side limiting", if the ORMQueryExecution tracer is set to level 4 (verbose). See ORM Support classes tracing for details on this tracer.
Paging through an entity collection is implemented in an overload of entityCollection.GetMulti(). The particular overload accepts the page size, which is the number of objects to retrieve in the fetch action, and the page number to retrieve.
If you for example pass 10 for the page size and 4 for the page number, you'll get record number 31-40, the first record is 1, the first page is also numbered 1. Paging is disabled if you pass 0 for the page number or 0 or 1 for the page size.
When using paging, it's often required to know how many pages a given resultset contains.For example, you want to show a list of page numbers the user can choose from, like Google uses. You can retrieve the number of objects matching your filter by using the entity collection method entityCollection.GetDbCount().
The method has overloads to support filtering spanning multiple entities as well. Below is an example to retrieve the total number of different order objects of customers from "France".
var orders = new OrderCollection(); var relations = new RelationCollection(); relations.Add(OrderEntity.Relations.CustomerEntityUsingCustomerId); int amount = orders.GetDbCount(CustomerFields.Country.Equal("France"), relations);
The number is retrieved by using the GetScalar() method of the entity
collection class, using the
Count(*) aggregate function. See for more
details about using Aggregate functions in your code the section Field
expressions and aggregates.
The value in amount can now be used to calculate the total number of pages when the page size is given: number of pages = (number of objects / pagesize) + n, where n is either 0 (total number of objects modulo pagesize is 0) or 1 (total number of objects modulo pagesize > 0). Below is the code to retrieve page 4, with a pagesize of 10 objects. We re-use the filter objects used in the GetDbCount() call:
orders.GetMulti(filter, 0, null, relations, 4, 10);
After this call, orders contains 10 objects, which formed the 4th page in the result set matching the filter defined. No sorting is applied here, but if you specify a sort expression, the sorting is performed prior to the paging logic.
It's recommended to use a sorter in your query if you use paging to be sure the data is ordered in a predictable fashion. SQL by definition applies no ordering on SELECT resultsets.
The paging functionality is also available for typed list and typed view classes, through an overload of the Fill() method. For typed lists and typed views, the same definitions are valid as for collections: page numbers start at 1, the first record is numbered 1 and paging is disabled if you pass in a page number of 0 or you pass in a page size of 0 or 1.
Getting the total number of rows for a typed list or typed view works the same as it is done for a collection. A typed list and typed view have a method called GetDbCount() and various overloads accept a filter, and for typed lists als a RelationCollection. You can also specify if the GetDbCount() should take into account duplicate rows or not.
Fetching a given page in the typed list or typed view is then boiling down to using the Fill() overload which accepts the two paging parameters. Be sure to clear the typed list/typed view object before calling Fill() again to fetch another page.