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ReadOnly Annotation

Use Case:-

  1. You want to show up to 10000 record on single VF page.
  2. Count of records based upon some business requirement where number of records could go up to 1 million.
So far, it was not possible to achieve above in VF page because of following limitations:-
  1. The maximum number of items in a collection that can be iterated over using components such as <apex:dataTable>, <apex:dataList>, and <apex:repeat> is 1000.
  2. Normally, queries for a single Visualforce page request may not retrieve more than 50,000 rows.
Solution:-

But with API version 23.0, salesforce has introduced 'ReadOnly' annotation which has following functionality/restriction:-
  1. The @ReadOnly annotation allows you to perform unrestricted queries against the Force.comdatabase. All other limits still apply. It's important to note that this annotation, while removing the limit of the number of returned rows for a request, blocks you from performing the following operations within the request: DML operations, calls to System.schedule, calls to methods annotated with @future, and sending emails.
  2. The @ReadOnly annotation is available for Web services and the Schedulable interface. To use the @ReadOnly annotation, the top level request must be in the schedule execution or the Web service invocation. For example, if a Visualforce page calls a Web service that contains the @ReadOnly annotation, the request fails because Visualforce is the top level request, not the Web service.
  3. Visualforce pages can call controller methods with the @ReadOnly annotation, and those methods will run with the same relaxed restrictions. To increase other Visualforce-specific limits, such as the size of a collection that can be used by an iteration component like <apex:pageBlockTable>, you can set the readonly attribute on the <apex:page> tag to true. 
Below is sample visualforce page & controller demonstrating "readonly" in action:-
  1. Controller class:-                                            
  2. VisualForce Page:-                                            



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