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Published Friday, July 3, 2009 4:58 PM by Greg Low

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Saggi Neumann said:

Hi Greg,

I too don't know enough about Oracle, but as an anecdote, I lately discovered that if developers find out about the Database Tuning Advisor, you can quickly find yourself in bizarro world where indexes *are* superfluous (and include most of the columns as well just for the heck of it), and getting rid of them isn't an easy task. It's actually harder than adding indexes to a database full of heaps.

There are roles in msdb to allow running and managing jobs, to manage DTS packages and to send mail, but there's no role to disable DTA, right? So I googled "disable database tuning advisor" and the first result to show was this:

"Oracle Database automatically runs the SQL Tuning Advisor on selected high-load SQL..."

So - I'm not saying that indexes are superfluous by default in Oracle and I'm not saying that they aren't, but if Microsoft, the leading "we'll do it for you" company doesn't do automatic index tuning, then how come Oracle who only recently found out about GUI does that?! ;-)


S. Neumann

July 3, 2009 1:36 AM

Greg Low said:

Yes, I feel your pain. I've been to a number of sites where they run DTA blindly and just accept the recommendations. It's often quite a mess after a while and with statistics as well, not just indexes.

The missing index DMVs can also lead you down that path, particularly given their focus on an individual query rather than on a query load.

But still more commonly, I go to sites where the devs "just haven't yet gotten around to doing any indexing" and that's often not pretty.

I was at a site last Tuesday that had 20G tables without any sort of index or structure to the tables at all. Not surprisingly, queries to find entries from the last thirty days were somewhat slow :-)



July 3, 2009 1:55 AM

Mike Walsh said:

I have actually been reading his "Art of SQL" book and finding it a good book to give to developers to get into a better mindset around thinking of performance, thinking of sets, etc. Haven't finished it yet but was going to finish it and think about the next book. Might hold off on checking this one out.

I found him when looking for an analogy on join types and found some of his you tube videos to be a handy reference to give to some developers I work with.

July 6, 2009 8:00 PM

Kevin G. Boles said:

+1 on the DTA-is-often-horrible mantra!  Earlier this year I spent almost 150 manhours doing an index cleanup project for a client that blew out tons of DTA-recommended indexes over the course of 12-14 months.  With the increase in data volumes their applications became extremely slow due to excessive blocking and DML index maintenance activity.  I was able to remove over 60% of all indexes with less than 5% reduction in read performance but > 200% improvement in overall DML response time all while reducing database space used by a very significant amount.  DTA sure does love to include columns for 'covering' queries!!  

July 7, 2009 10:48 AM

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