A look at an Oracle instance

When you install Oracle, on a very high abstraction level, you copy files to your HD. Nothing more nothing less.

After this process, you got an Oracle installation: a system, with at least one directory where Oracle’s programs reside, but you don’t have any Instance.

An Oracle Instance is the software (and memory) that Oracle uses to manipulate the data in the database. An Instance can by identified by its Oracle SID (System IDentifier) and is made up of Oracle system files (like the Init file which is where all Oracle’s parameters resides, and files where Oracle store its own structures and internal data) and of user files (files that store your data). An Instance (when running) consume RAM and CPU.

Every Oracle’s Instance has got:

- A SID. SIDs must be unique inside a Server

- An Init<SID>.ora. The Init file with all Oracle’s parameters inside, its complete name is like that (you must substitute your SID to <SID>, so if you got a SID equal to MYORA the Init would be InitMYORA.sid)

- Some memory assigned to

- A bunch of system data file like the system.ora data file (it’s like a user data file but it stores only Oracle’s own data)

- A bunch of user data file

Oracle, what?

Oracle is a relational data base (DB).It is a program that use the relational model (proposed in 1969 by Edgard Codd) based on predicate logic and set theory.

Table’s data are treated as sets: so you can have the set of users, the set of employees, the set of books, the set of cars etc.

“Links” between these sets are logical “operation”, are relations between them, and define new sets: the set of books read by users, the set of cars owned by employees.

For the rest of us: Oracle is a system that can read and write data in a very efficient way. What Oracle does is to use files in a very efficient way and to hide this complexity to the final user…to you!

Each time you write a query and access a set (that is read a table’s data) you are telling Oracle to read at least a file.

Each time you write an “expression” that modify a set (that is write, change or delete table’s data) you are telling Oracle to read at least a file.

That’s more or less what Oracle do!

An Oracle blog on the basic

Yep!

Finally I found a motivation to work again on this blog.

The idea is to write on basic Oracle concept and to create a kind of “Introduction to Oracle”.

Hope that it will help.

A new Begin

After more than one year I decide to restart this blog!

Now, here I want to talk about Oracle basic concepts.

Hope to bring a bit of fun in Oracle Universe! ^_^

So this is a new Begin, with the hope that it will not be just:

Begin
Null;
End;
/

Multitable Insert

So, let’s start with two normal inserts:

insert into tab1
select 1,1,’first’ from dual

and

insert into tab2
select 1,1,’first’ from dual

this work fine, but we got Oracle 9 and 10 nowadays so we can:

Insert all
into tab1
into tab2
select 1,1,’first’ from dual

and this give us:
ORA-00918: column ambiguously defined

A bit strange.

Explain me!

This query is slow!

(copyright by my Boss)

But how can you examine what’s going on? Can you “see” what Oracle is thinking?

Yes you can!

V$iews

Oracle, as any other DBs, has got a Catalog: internal tables and views that give you information about tables, users, sessions and so on.

One subset is the v$ views (v$iews for short) subset.

Hello world!

So here we go!

First Blog! First Post! First all!

More  or less the start of an adventure!