Re: Block Header Size Confusion

On Nov 25, 11:14 pm, "Ed Prochak" <edproc..._at_gmail.com> wrote:
> On Nov 25, 4:01 pm, "Dereck L. Dietz" <diet..._at_ameritech.net> wrote:
>
>
>
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> > 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?
>
> > 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).
>
> > Thanks.As I recall, in Oracle 8 it was a guestimate. If you had a decent
> estimate of the row size, then you get a first pass extimate of the
> rows per block. Computing the overhead then might cause the row count
> to change.
>
> So you are right in seeing the catch-22, but it isn't a fatal
> situation.
>
> HTH,
> Ed- Hide quoted text -- Show quoted text -

>> am I correct in thinking it is referring to the number of rows in the data BLOCK and not the entire table? <<

Yes, you are. Out of curiousity whose study guide are you referring? The Oracle 9.2 Concepts manual says that if a data block is empty but once held 50 rows that the row directory would be 100 bytes in size: Oracle9i Database Concepts Release 2 (9.2) Part Number A96524-01, Ch 2, Data Blocks, Extents, and Segments under the heading label Row Directory.

HTH -- Mark D Powell -- Received on Sun Nov 26 2006 - 14:47:28 CST