Bulk Data | Visual Force | readOnly

Working with Large Sets of Data in Visualforce Page

Visualforce iteration components, such as<apex:pageBlockTable> and <apex:repeat>, are limited to a maximum of 1,000 items in the collection they iterate over.

Sometimes your Visualforce pages may need to work with or display larger sets of data, but not need to make modifications to that data;
for example, if you are providing custom reporting and analytics. Visualforce offers developers a “read-only mode”, which relaxes the limit on the number of rows which can be queried in one request, and increases the limit on the number of collection items which can be iterated over within the page.

Setting Read-Only Mode for an Entire Page
To enable read-only mode for an entire page, set the readOnly attribute on the <apex:page> component to true.

For example, here is a simple page that will be processed in read-only mode:
<apex:page controller="SummaryStatsController" readOnly="true">
    <p>Here is a statistic: {!veryLargeSummaryStat}</p>
</apex:page>
The controller for this page is also simple, but illustrates how you can calculate summary statistics for display on a page:

public class SummaryStatsController {
    public Integer getVeryLargeSummaryStat() {
        Integer closedOpportunityStats =
        [SELECT COUNT() FROM Opportunity WHERE Opportunity.IsClosed = true];
        return closedOpportunityStats;
    }
}

Note:
1. In read-only mode, total number of records retrieved by SOQL queries is up to 1 million rows.
2.With readOnly attribute maximum number of items in a collection that can be iterated over using    components such as <apex:dataTable>, <apex:dataList>, and <apex:repeat> is increased from 1,000 items to 10,000.
3.Read-only mode for the entire page can’t use data manipulation language (DML) operations.