[askdba] Re: Need help with Interview Questions

  • From: Nisar Tareen <ntareen@xxxxxxxxx>
  • To: askdba@xxxxxxxxxxxxx
  • Date: Mon, 8 Nov 2004 13:42:01 -0800 (PST)



I did not recall of mentioning Parallelism what I rote to Irfan, My answer was 
about multiple extents. 


Yes, I will stand on my ground that multiple extents (as I said extra ordinary) 
will affect the performance for full table scan, as the disk head had to 
reposition itself

To make extra hits (I/O)  for read and write/update, and extra block addresses 
in SGA for the records read.


My understanding my logic, Ganesh most of the time I write from the top of my 
mind, After doing this work for so long, I do not go through the technical 
manual, My idea is not to write technical manual for every email or query, 
Email should give the major guide line on the issue and each one of us the user 
should go the extra 9 yards according to our needs.  


Believe me I ran Tara byte database and I don't have to prove, I had done my 
proving, Few extents are better that many. 


I am not an academic and I am running a shop and the time I have I enjoy 
answering emails and some time for fun add my logic.  But Oracle is in the 
business of proving yes I can be wrong then you just have to say, Nisar here 
you are wrong. I will and you can give the understanding on an issue, its okay 
with me. 


Plus,  extents are Physical blocks not logical.  eg. 1 block of 10 giga bytes 
or 10 blocks of 1 biga bytes. 


Have Fun. 

Nisar Tareen 

Ganesh Raja <ganesh.raja@xxxxxxxxx> wrote:

Did not Understand too much from your Post .. But How does all the
things that you have talked about affect the way parallelism works ..
Parallelism is CPU Bound and not disk bound ... U dictate if you want
to Use Parallelism and then The ur Query/DML is Split across multiple
processess that works out how to fetch data from the database and
processes it.

I am disagreeing here with irfhan also when he says that Multiple
extents will give u chances of better parallelism. U can write a small
testcase with a One Extent Table and See what happens .. U will still
get Parallel Reads.

The Only thing that has some effect on performance with Multiple
Extent is the MBRC value since an Multi Block read cannot be done over
multiple extents. [ U will never encounter this has a Problem]

Apart from that i will never say that if you have 1000 Extents ur
Query is Going to Perform badly and if you have 1 Extent it will
perform any better. I need a test case from u to prove that the

Extent has always been a Logical Grouping of Blocks and not a
Physicall Grouping.

My $0.02 and not my logic ... :)


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