If you start a read-only transaction, you can retrieve a snapshot
    of the database at the moment the transaction started. Other
    users can update records in the relation you are using, but your
    transaction retrieves the records as they existed at the time
    the transaction started. Any changes that other users make and
    commit during the transaction are invisible to you. A read-only
    transaction lets you read data without incurring the overhead
    of record locking. You cannot modify, store, or erase records or
    execute data definition statements in a read-only transaction.