RE: DBA Interview Questions

  • From: "Freeman, Donald" <dofreeman@xxxxxxxxxxx>
  • To: <ryan_gaffuri@xxxxxxxxxxx>, "Oracle-L (E-mail)" <oracle-l@xxxxxxxxxxxxx>
  • Date: Wed, 24 Nov 2004 15:33:34 -0500

This is a pretty small shop.   We have two pairs of DBAs. One pair each in our 
transactional and data warehouse environments.  One full time Health Dept 
employee married up with a contract DBA.  I have six instances on 4 servers and 
they are all small.  Prod is about 50G.  We're not quite into maintenance but 
95% of the development is done. So, we don't really need a coder.  All our ETL 
is done using Informatica and we have 2 full time experienced Powermart users.  
All our reporting is done using Cognos and we have 2 full time Cognos 
developers.   And, as far as it goes, they are all pretty good at their jobs, 
and within the limits of the tools they are using they write good code.  We 
have no performance issues.  We are using Oracle OMS for job scheduling and are 
having an ongoing problem with a bug causing our daily load job to hang.  It 
wasn't fixed in 9.2.0.6.  I need someone who can stay and do stuff during the 
bi-weekly maintenance periods, weekends etc.  Although we can hire another DBA 
we can't pay me overtime because, "<insert some reason here>."
 
The contract DBA will be doing light developer support and maintenance.  Thats 
about it.  Going forward we are going to RAC, SAN, maybe 10G this year (05), 
Data Guard and disaster recovery site in 06.   I want a guy hired who knows 
something that I don't know in order to further train me and to provide 
something approaching 24/7 coverage.  
 
I don't really have much hope that the average DBA is going to know about wait 
interface tuning.   I asked this guy today what resources he could call on to 
find help and information and the best I could get out of him was 'Google.'   I 
personally think there is a plethora of good and bad information available and 
its important where you get it.  If somebody said that they were an avid reader 
of this list, again, I would be impressed. 
 
I arrived at this list because I bought the Milsap & Holt performance tuning 
book, the list subscription instructions were in the book, and the 
troubleshooting principles espoused in the book were similiar to the methods I 
used during my previous career as a radar tech.  I'm not sure how far that 
information has penetrated into the Oracle community. 
 
There is a whole lot more to Oracle but not a great deal more to this job.  We 
will not be getting a face to face interview.  The contractor is responsible 
for filling the position and most of these guys have a history of successful 
contracts with the contractor.  From their perspective the applicants are a 
known quantity and we'll simply try to pick the best available.  If they don't 
work out, we'll get another one. At least 50% of this job will involve getting 
along with me<g>. 
-----Original Message-----
From: ryan_gaffuri@xxxxxxxxxxx [mailto:ryan_gaffuri@xxxxxxxxxxx]
Sent: Wednesday, November 24, 2004 3:08 PM
To: dofreeman@xxxxxxxxxxx; Oracle-L (E-mail)
Cc: Freeman, Donald
Subject: Re: DBA Interview Questions


 

is this a developer (coder) or a dba spot or a hybrid? also what is most 
important... someone very good at datawarehousing and ETL tools or someone 
intimately familiar with the database who can tune? If its the latter, you will 
have a better shot grabbing a production DBA and having the person write 
PL/SQL. Warehouse jobs typically pay well so it may not be as hard as it would 
be to grab a DBA and have the person do OLTP code. A good selling point for a 
prod DBA also is that data warehouse experience is far more valuable than OLTP 
experience in todays marketplace and its hard to break into the warehouse 
market. 
 
 its very hard to find a programmer who knows much about tuning. we had to 
reject 95% of our phone screens to get people who can do it here... and we are 
near DC with a large pool of applicants. Can't hire consultants so that does 
cut down on the top candidates somewhat. 
 
are you using RAC? if not, doesn't matter if they know it. 
 
Some questions that might help:
1. when do you use a bitmap index(since you have a warehouse). this is 
interesting because the oracle documentation is wrong. Most people recite the 
docs. Jonathan Lewis has some excellent articles about this on dbasupport(I 
think that is the write one...). 
2. in an explain plan what does cardinality and bytes mean and in what cases 
would either be useful? Cardinality is more useful when tuning oltp type 
queries and bytes is more useful when tuning bulk processing. 
3. the true senior question on the 10046 trace(curteous of cary milsaps book) 
what does it mean the your timing values = 0 in the 10046 trace? it means 
time_statistics= false and you need to turn it to true and retrace. 
4. for datawarehousing being able to interpret a query plan in advance of 
running the query is very important since queries can run for hours(even if 
they are optimized). I think the 10046 trace is less important than reading a 
query plan in your case. I would focus more on that. It's very hard to tune a 
query that takes hours based strictly off of a plan especially complex onces... 
5.  basic one, how do you speed up bulk processing? See if they atleat know to 
turn off constraints and triggers, etc... 
6. what does nologging mean? In a warehouse you will probably use that alot. 
 
if you find someone who has read dan tows sql tuning book and uses his 
strategy, you probably found a sql pro... 


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