Re: Three simple questions

Hi Arthur,

    Embedded you will find my simple answers to your questions.

Arthur Merar wrote in message <35ECA27D.6D25983F_at_unsu.com>...
>
>Hello,
>
>I have three simple questions about Oracle I am trying to understand
>being a newbie.
>
>1) What does it mean when Oracle is 'coalescingfree space'?

    When rows are deleted from a table the space they occupied is not reclaimed. This is accomplished by coalescing (compacting) the data in a separate operation. (It isn't done at the time the data is deleted due to potential performance impacts.)
>
>2) I am not sure I understand bitmap indexing.

    Bit-mapped indexes are used to overcome the inherent inefficiency of the traditional b-tree indexes when dealing with columns with low cardinality. For example, suppose you want to look at your data based on column that can only have the values C, D, and N. Given an even distribution approximately 1/3 of the rows would have each value and a b-tree index on the column would be useless. With a bitmapped index three bits would be assigned in the index to each row in the table. For each set of three each bit would correspond to one of the three values. The benefit is that instead of doing a full table scan, that is, reading every row in the table it is only necessary to read the index in order to build a candidate result set list. Assuming even a short 80 byte row the use of the bit-mapped index would reduce the I/O by a factor of 200 -- yes 200!

>
>3) The book I have says that when a rollback segment grows larger than
>the 'optimal' size, it shrinks it back to optimal size. If that is
>true, what does it do with the rollback data it must delete in order to
>shrink the segment?

    Well, the book forgot to mention that the rollback segment isn't shrunk until after the operation is completed.

regards

Jerry

>
>Thank you for your help.
>
>--
>
>"A real friend is someone who knows all about you and still respects
>you."
>
>Arthur
>amerar_at_unsu.com
>ICQ#:663601
>http://www.unsu.com
>
>
Received on Wed Sep 02 1998 - 13:14:25 CDT