How do I set the isolation level for SELECT statement?
You don't need to do anything to get statement level consistency

A query always sees a consistent state of the database regardless of the isolation level you use. 

Quote from the manual: 


  This means that each SQL statement sees a snapshot of data (a database version) as it was some time ago, regardless of the current state of the underlying data. 


See the chapter about Multi Version Concurrency Control in the manual