Re: Oracle basics

From: Sybrand Bakker <postbus_at_sybrandb.demon.nl>
Date: Thu, 20 Sep 2001 20:48:40 +0200
Message-ID: <tqkeglg5lfc961@news.demon.nl>

Comments embedded.

Regards,

Sybrand Bakker, Senior Oracle DBA

"Anon" <Anon_at_invisible.com> wrote in message news:hSmq7.74724$bY5.353715_at_news-server.bigpond.net.au...
> Hello,
>
>
>
> I have a quick. I can't seem to find a deffinative answer on this and I am
> sure that there are a number of other people who must also be suffering
the
> same confusion.

>
>
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> Basically, I need some help understanding the structure of Oracle objects,
> both physical and logical.
>
>
>
> People seem to refer to a database as a physical set of files that form a
> working data container and objects for Oracle. i.e. control files, redo
logs
> & data files. I repeatedly see the fact that you can not see a tablespace
at
> OS level as it is a logical structure, not physical. However, when I issue
> the following command:
>
>
>
> create tablespace test datafile
>
> '/apps/oracle/data/test.dbf' size 200M
>
> default storage (
>
> initial 8k
>
> next 256k
>
> pctincrease 0
>
> minextents 1
>
> );
>
>
>
>
> It does create a physical file which I can populate with tables, views
> etc...
>
>
>
> At this point I have not issued the create database command and therefore

NO, FALSE
You did create a database way before you ever created this tablespace

Look at the following picture

         Physical

                   database


SYSTEM tablespace (without this no database)       other tablespaces

|
|
v
1 or more datafiles per tablespace

Logical

                 database
                    |
                    |
                    |
                    v
          1 or more schemas (this is what is called in sqlserver lingo:
'database'
          Microsoft and Sybase didn't implement the 3 layer database model)
                   |
                   |
                   v

          1 or more database objects

The *physical* implementation of an object is called a *segment*

a *segment* resides in a ...... YES, tablespace

a *segment* consist of multiple *physical* extents.

Of course this is just an abstract of the discussion in the Oracle Concepts Manual which everyone coming from sqlserver or sybase uses as if it were the Bible or the Red Book of Mao Ze Dong.

> wonder where it comes in to play and why I haven't had to issue the create
> database command. Is it to do with the default storage line ? Is this
> creating a database for me without my knowledge ?
>
>
>
> If this is the case, then you would draw the conclusion that a tablespace
is
> a logical division of a physical database. Is this correct ?
>
>
>
> Any clarity on this would be appreciated.
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Received on Thu Sep 20 2001 - 13:48:40 CDT