Oracle FAQ Your Portal to the Oracle Knowledge Grid

Home -> Community -> Usenet -> c.d.o.server -> Re: table size not true with dba_segments??

Re: table size not true with dba_segments??

From: Charles Hooper <>
Date: 28 Mar 2007 05:42:01 -0700
Message-ID: <>

On Mar 28, 8:08 am, "Carlos" <> wrote:
> On 28 mar, 13:47, "ffanchy" <> wrote:
> > On my opinion, in each case, you can do better job manually than using
> > automatic management of extends.
> > However unless you want to spend at least 1 day per week managing all
> > this it is quiet better to use automatic management of extend.
> > I am using it on an OLTP database with arround 2000 tables and 150 Go
> > RAC on 9i database. It is working fine.
> > Automatic management is using bitmap to manage extends instead of
> > dictionnary, it is fastter and recommanded by Oracle.
> > (If you are creating a new database try also to use the automatic
> > freelist management parameter EXTEND MANAGEMENT LOCAL.
> > With this parameter only the PCTFREE parameter has to be set all the
> > others are managed by Oracle. This let you time for other stuffs.
> > It is also quicker as freelists are manage by bitmap in the first
> > block of the extend ).
> I think you are a little bit confused between LMT (Locally Managed
> Tablespaces) and ASSM (Automatic Segment Space Management). LMT have
> nothing to do with freelists. (Ah! And LMT manage EXTENTS, no
> "extends").
> Cheers.
> Carlos.

Carlos is of course correct. Locally managed tablespaces are preferred to dictionary managed tablespaces, as there is less performance impact from the overhead required for Oracle to maintain locally managed tablespaces compared to dictionary managed tablespaces. Additionally, locally managed tablespaces can be created such that all objects placed into the tablespace have a consistent extent size, regardless of the extent parameters that are specified for the objects.

ASSM removes the need for managing freelists, as well as managing extent sizes. As additional extents are added for objects, the new extent increase in size in a predictable fashion. One of the downsides of ASSM is that it can potentially hurt performance, as it artifically increases the clustering factor of indexes due to the way that it decreases insert contention on the objects placed into such tablespaces. There are many great articles that decribe this behavior in greater detail.

Charles Hooper
PC Support Specialist
K&M Machine-Fabricating, Inc. Received on Wed Mar 28 2007 - 07:42:01 CDT

Original text of this message