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From the thread What is the difference between sys and system accounts in Oracle databases?, I understand that sys schema is supposed to be used only by the database.

But still an administrator can change the password for the schema.

Can the database connect without the password? If not, how does it connect even after the password is changed?

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Please clarify, are you asking about a sqlplus session without password, in the form: "sqlplus / as sysdba" or are you talking about just the normal operation of the database? – REW Jan 11 '11 at 5:09
up vote 9 down vote accepted

The database doesn't bother authenticating itself to itself.

As the database does not use the password identification strategy when performing operations, changing the SYS password only impacts how humans can use the account.

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At the shell prompt, on the machine your database runs on, as the Unix user your Oracle runs as, try:

$ sqlplus / as sysdba

No password is necessary. What is actually happening is that your session attaches directly to the shared memory that is your instance relying only on Unix permissions - there is actually no way for Oracle (or any process) to stop you doing this type of operation, so rather than fight it, they made it a feature.

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yeah , you can conncet to database with no passwprd when you conncet as admin on serever :

$ sqlplus
username : sys as sysdba
password : (empty)

then the database open with idle . you can work with idle instance for some of activities

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