[askdba] Re: Need help with Interview Questions

  • From: Ganesh Raja <ganesh.raja@xxxxxxxxx>
  • To: askdba@xxxxxxxxxxxxx
  • Date: Tue, 9 Nov 2004 10:21:04 +0000

Irfhan had given a Good explain there and i thought i will not again
go back to bickering about this .. But nisars comments on Disk Head
movement and he considering an Extent as a Physical and not a Logical
Component has spurred me to write this. [ i dont want anybody to think
otherwise on this ]

1. A Block is a physicall storage and it may consists of mutiple OS Blocks .. 
2. A Block is never contigous.
3. Blocks within an Extent are never contigous

So now let us see where we stand with disk head and placement. :)


On Mon, 8 Nov 2004 13:42:01 -0800 (PST), Nisar Tareen <ntareen@xxxxxxxxx> wrote:
> Ganesh,
> 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:
> Nisar,
> 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
> otherway.
> Extent has always been a Logical Grouping of Blocks and not a
> Physicall Grouping.
> My $0.02 and not my logic ... :)
> Rgds
> Ganesh
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