Need for the Extent storage structure

[khemani]

Hi,

I am trying hard to understand the Oracle database architecture. It is mainly due to my lack of knowledge, I suppose, that I cannot seem to understand the need for the Extent logical storage structure. Is it cumbersome to talk directly in terms of data blocks? Does dealing with extents vs dealing with data blocks directly reduce the storage management overhead for Oracle? What is the rationale behind having this intermediate storage structure between the segment and the data block? Please clarify.

Thanks,
Karim

[Thomasps]

http://download-west.oracle.com/docs...o.htm#sthref53

[slimdave]

It allows for growth of the segment in stages, where each stage is a contiguous set of blocks for that segment. The alternative would be to have individual blocks scattered around, which would be very inefficient to read with a full table scan and require another large structure to tell oracle where each one is.

[abhaysk]

Dave.. but do we know if the extents will be contigious for sure with RAID?.. if not we still have issues with FTS.. dont we?

Extents is a logical structure which will want the blocks in it to be contigious but cannot be assured how OS allocates in disk..

The rationale behind extent is to simplify the understanding of allocation process in a datafile/s.. and ofcourse it will be contigious most of the times and hence making the FTS process efficient.


-Abhay