Re: Large tables, updates, selects and varchars

On Apr 3, 2:10 pm, chrism..._at_gmail.com wrote:
> On Apr 2, 9:02 pm, DA Morgan <damor..._at_psoug.org> wrote:
>
>
>
> > chrism..._at_gmail.com wrote:
> > > I am using Oracle 10g (10.2.0.3.0) and have a large table (1+ billion
> > > rows). It currently has about 15 columns in it. I have a requirement
> > > for a new varchar2(4000) column to go with the current data in that
> > > table. I need to update this column after it has been added to the
> > > table. I have been told that I may be better off putting the column
> > > in a separate table because by adding it to an existing table, Oracle
> > > has to jump around the hard drive to update it and find it, and
> > > therefore this will degrade performance. Of course, by putting it in
> > > a separate table, a join will be required on selects.
>
> > > From a purely design point of view, it makes a lot more sense to add
> > > the column to the existing table, but I don't know the internals of
> > > Oracle. When modifying existing tables on large Oracle databases, do
> > > you generally have to be wary of what columns you add?
>
> > > I can't really test this ahead of time--the update will take a very
> > > long time to perform, because there are many rows and a calculation is
> > > involved.
>
> > > Thanks.
>
> > 1+ billion rows with or without partitioning?
>
> > Is this the last change that will ever take place or part of what may
> > be ongoing modifications?
>
> > How is the new column going to be used? Will most queries need to
> > access the new data? Why VARCHAR2(4000) and not CLOB?
> > --
> > Daniel A. Morgan
> > Oracle Ace Director & Instructor
> > University of Washington
> > damor..._at_x.washington.edu (replace x with u to respond)
> > Puget Sound Oracle Users Groupwww.psoug.org
>
> We are partitioning with between 35 million and 60 million rows per
> partition. This probably will not be the last change we are going to
> make. There are likely to be future modifications.
>
> It's going to store file paths and most queries will need to access
> it. We chose VARCHAR2 because the size of the data will never exceed
> 4000 chars, and we assumed that CLOBs were less efficient than
> VARCHAR2s. Is this not the case?

I believe Daniel became suspicious because 4000 is the limit for VARCHAR2. Often this is used as replacement for CLOB. If your data is large text then a CLOB is probably more efficient. I doubt that it is in your case. It all depends.

AFAIK file system path name limits are far less than 4000 so you might as well use 1024 or something.

Kind regards

robert Received on Thu Apr 03 2008 - 08:21:51 CDT