Retrieves a snapshot of the database at the moment the read-only
    transaction starts. Other users can update rows in the table
    you are using, but your transaction retrieves the rows as they
    existed at the time the transaction started. You cannot update,
    insert, or delete rows, or execute data definition statements
    in a read-only transaction with the exception of declaring a
    local temporary table or modifying data in a created or declared
    temporary table. Read-only transactions are implicitly isolation
    level serializable.

    Because a read-only transaction uses the snapshot (.snp) version
    of the database, any changes that other users make and commit
    during the transaction are invisible to you. Using a read-only
    transaction lets you read data without incurring the overhead of
    row locking. (You do incur overhead for keeping a snapshot of the
    tables you specify in the RESERVING clause, but this overhead is
    less than that of a comparable read/write transaction.)

    Because of the limited nature of read-only transactions, they are
    subject to several restrictions.