Managing limits
Normally, the total number of records retrieved within a single controller execution context is 50,000 records (in total across all SOQL queries executed). In addition, Visualforce components such as apex:dataTable and apex:repeat can only iterate over 1000 items before a runtime exception is thrown.
At the time of writing this, the readOnly attribute (specified on the apex:page component) changes this to 1 million records and 10,000 iterations within Visualforce components on the page (refer to the Salesforce documentation for the latest update). As the name suggests, no updates to the database can occur, which basically means no DML at all. Note that queries are still governed by timeouts.
Tip
If you require a more granular elevation ...
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