If you just want to prevent any other session from modifying the data you can issue
LOCK TABLE whatever
/
This blocks other sessions from updating the data but we cannot block other peple from reading it.
Note that in Oracle such table locking is rarely required, because Oracle operates a policy of read consistency. Which means if we run a query that takes fifteen minutes to run the last row returned will be consistent with the first row; in other words, if the result set had been sorted in reverse order we would still see exactly the same rows.
For more details: http://www.cs.kent.edu/~wfan/link/dbapre/dbatest/54920f.htm
