DB Links

Database Links cabn be created as PUBLIC or PRIVATE.

create public database link "dblink name" connect to "target db user" identified by "target db user password" using 'sid';

create database link "dblink name" connect to "target db user" identified by "target db user password" using 'sid';

DB Links use Oracle Networking from the installed oracle home.

Public Database Links will be used before private DB Links.

usage:

select * from dual@dblink

Password File

Create Password File.
orapwd file=password_file_name password=the_secret_password
orapwd file=password_file_name password=the_secret_password entries=n

Add users to password files:
grant sysdba to "user"
grant sysoper to "user"

View users who have SYSDBA or SYSOPER priv:
select * from v$pwfile_users;

Connect:
sqlplus ""user" as sysdba"

note:
in 9.2, connection info shows connection as sys.
in 8.1.x, connection info shows connection as self.

remove db parameter from spfile

Required when obsolete db parameter needs to be removed or reset from the spfile.

alter system reset "parameter name" scope=spfile sid='*'

clustering factor

Clustering Factor shows how synchronized the indexes are. Clustering Factor is updated after running dbms_stats on a particular index.

A good clustering factor is when the clustering factor is near the number of blocks in the index.
A bad clustering factor is when the clustering factor is near the number of distinct rows in the index.

So for each key,
If the child records are found in the same data block then access will be quicker. Good clustering factor.

If the child records are found in may data blocks then access will be slower. bad clustering factor.

tracing

More information on tracing.

Use dbms_system to trace sessions ...

Turn on level 12 tracing
exec sys.dbms_system.set_ev(sid, serial#, 10046, 12,'');

Turn off tracing
exec sys.dbms_system.set_ev(sid, serial#, 10046, 0, null);

To analyze with tkprof
tkprof

sys=yes sort='(prsela, exeela, fchela)'

where sys=yes, means data dictionary queries are included.