4. Describe the Oracle Wait Interface, how it works, and what it provides. What are some limitations? What do the db_file_sequential_read and db_file_scattered_read events indicate?

 

The Oracle Wait Interface refers to Oracle's data dictionary for managing wait events. Selecting from tables such as v$system_event and v$session_event give you event totals through the life of the database (or session). The former are totals for the whole system, and latter on a per session basis. The event db_file_sequential_read refers to single block reads, and table accesses by rowid. db_file_scattered_read conversely refers to full table scans. It is so named because the blocks are read, and scattered into the buffer cache.

 

5. How do you return the top-N results of a query in Oracle? Why doesn't the obvious method work?

 

Most people think of using the ROWNUM pseudocolumn with ORDER BY. Unfortunately the ROWNUM is determined before the ORDER BY so you don't get the results you want. The answer is to use a subquery to do the ORDER BY first. For example to return the top five employees by salary: SELECT * FROM (SELECT * FROM employees ORDER BY salary) WHERE ROWNUM

 

6. Can Oracle's Data Guard be used on Standard Edition, and if so how? How can you test that the standby database is in sync?

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