Oracle FAQ Your Portal to the Oracle Knowledge Grid
HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US
 

Home -> Community -> Usenet -> c.d.o.server -> Re: Block Header Size Confusion

Re: Block Header Size Confusion

From: Brian Peasland <dba_at_nospam.peasland.net>
Date: Mon, 27 Nov 2006 01:58:36 GMT
Message-ID: <J9D9Hr.BKx@igsrsparc2.er.usgs.gov>


Dereck L. Dietz wrote:
> In reading a study guide for Oracle 9i it described the data block headers.
> One item it mentions is the Row Directory which it states occupies (4 * the
> number of rows) bytes.
>
> The other book I used to get the formula I have been using to "guestimate"
> table/index sizing was the Oracle 8 DBA Manual and it just had the block
> overhead as 90 for tables and 161 for indexes.
>
> My question is probably a simple one and I apologize if it is but when it
> states (4 * the number of rows) am I correct in thinking it is referring to
> the number of rows in the data BLOCK and not the entire table?

The number of rows in the block, not the entire table.

> Another question is, isn't this a Catch-22 situation? It seems that in
> order to know what the block overhead is you need to know how many rows will
> be in the block; but in order to know how many rows will fit in the block
> you need to know the free bytes in the block (block size minus the block
> overhead).

Keep in mind that Oracle actually starts adding rows from the end of the block...towards the block header. In this way, the block header can grow if needed.

HTH,
Brian

-- 
===================================================================

Brian Peasland
dba_at_nospam.peasland.net
http://www.peasland.net

Remove the "nospam." from the email address to email me.


"I can give it to you cheap, quick, and good.
Now pick two out of the three" - Unknown
Received on Sun Nov 26 2006 - 19:58:36 CST

Original text of this message

HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US