DBA Interview Questions
May 17, 2007
There are nearly an infinite number and combination of questions one can pose to a DBA candidate in an interview. I prefer to lean towards the conceptional, rather than the rote, as questions of this kind emphasize your foundation, and thorough understanding. Besides, I've never been one to remember facts and details I can lookup in a reference. Therefore, with that in mind, here are some brainteasers for you to ponder over.
1. Why is a UNION ALL faster than a UNION?
The union operation, you will recall, brings two sets of data together. It will *NOT* however produce duplicate or redundant rows. To perform this feat of magic, a SORT operation is done on both tables. This is obviously computationally intensive, and uses significant memory as well. A UNION ALL conversely just dumps collection of both sets together in random order, not worrying about duplicates.
2. What are some advantages to using Oracle's CREATE DATABASE statement to create a new database manually?
3. What are three rules of thumb to create good passwords? How would a DBA enforce those rules in Oracle? What business challenges might you encounter?
Typical password cracking software uses a dictionary in the local language, as well as a list of proper names, and combinations thereof to attempt to guess unknown passwords. Since computers can churn through 10's of thousands of attempts quickly, this can be a very affective way to break into a database. A good password therefore should not be a dictionary word, it should not be a proper name, birthday, or other obvious guessable information. It should also be of sufficient length, such as eight to ten characters, including upper and lowercase, special characters, and even alternate characters if possible.
Oracle has a facility called password security profiles. When installed they can enforce complexity, and length rules as well as other password related security measures.
In the security arena, passwords can be made better, and it is a fairly solvable problem. However, what about in the real-world? Often the biggest challenge is in implementing a set of rules like this in the enterprise. There will likely be a lot of resistance to this, as it creates additional hassles for users of the system who may not be used to thinking about security seriously. Educating business folks about the real risks, by coming up with real stories of vulnerabilities and break-ins you've encountered on the job, or those discussed on the internet goes a long way towards emphasizing what is at stake.
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-5 employees by salary:
SELECT * FROM (SELECT * FROM employees ORDER BY salary) WHERE ROWNUM < 5;
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?
Oracle's Data Guard technology is a layer of software and automation built on top of the standby database facility. In Oracle Standard Edition it is possible to be a standby database, and update it *manually*. Roughly, put your production database in archivelog mode. Create a hotbackup of the database and move it to the standby machine. Then create a standby controlfile on the production machine, and ship that file, along with all the archived redolog files to the standby server. Once you have all these files assembled, place them in their proper locations, recover the standby database, and you're ready to roll. From this point on, you must manually ship, and manually apply those archived redologs to stay in sync with production.
To test your standby database, make a change to a table on the production server, and commit the change. Then manually switch a logfile so those changes are archived. Manually ship the newest archived redolog file, and manually apply it on the standby database. Then open your standby database in read-only mode, and select from your changed table to verify those changes are available. Once you're done, shutdown your standby and startup again in standby mode.
7. What is a database link? What is the difference between a public and a private database link? What is a fixed user database link?
A database link allows you to make a connection with a remote database, Oracle or not, and query tables from it, even incorporating those accesses with joins to local tables.
A private database link only works for, and is accessible to the user/schema that owns it. A global one can be accessed by any user in the database.
A fixed user link specifies that you will connect to the remote db as one and only one user that is defined in the link. Alternatively, a current user database link will connect as the current user you are logged in as.
As you prepare for your DBA Interview, or prepare to give one, we hope these questions provide some new ideas and directions for your study. Keep in mind that there are a lot of directions an interview can go. As a DBA emphasize what you know, even if it is not the direct answer to the question, and as an interviewee, allow the interview to go in creative directions. In the end, what is important is potential or aptitude, not specific memorized answers. So listen for problem solving ability, and thinking outside the box, and you will surely find or be the candidate for the job.