1. Add an offset to primary key

Site 1 – create sequence test_seq start with 1 increment by 1;
Site 2 – create sequence test_seq start with 10000000 increment by 1;

Advantages:
a) Primary key datatype remains number

Disadvantages:
a) On site 2,3… – the primary key becomes too large (as size on disk)
b) Can have primary keys conflicts, over the time, if the start value for site 2,3.. is setting initially too low

2. Concatenate a unique identifier to the current primary key (eg: SITE name)

Advantages:
a) Can’t have primary key conflicts over the time
b) Simple to add another site
c) Can add unlimited number of site

Disadvantages:
a) Possible current application code changing to deal with this method
b) The primary key datatype becomes varchar2 instead of number

3. Composite primary key (worst than method 2 because it is necessary to change the current application code)

4. Intercalated sequences (the best method)

Site 1 – create sequence test_seq start with 1 increment by 10;
Site 2 – create sequence test_seq start with 2 increment by 10;

Advantages:
a) Can’t have primary key conflicts over the time
b) Simple to add another site
c) No current application code changing
d) Primary key datatype remains number with small size

Disadvantages:
a) Can add maximum 10 sites (the value of increment by)

5. Use SYS_GUID to generate primary key

Advantages:
a) SYS_GUID generates and returns a globally and randomly-dispersed unique identifier

Disadvantages:
a) RAW datatype instead of number (there are some RAW datatype restrictions)
b) Current application code changing

• A datafile can be associated with only one database.
• Datafiles can have certain characteristics set to let them automatically extend when the database runs out of space.
• One or more datafiles form a logical unit of database storage called a tablespace.

Data in a datafile is read, as needed, during normal database operation and stored in the memory cache of Oracle. Modified or new data is not necessarily written to a datafile immediately. To reduce the amount of disk access and to increase performance, data is pooled in memory and written to the appropriate datafiles all at once.

Sources and resources:

• Documentation
• Overview of datafiles
• Managing datafiles
• Moving datafiles
• Datafile changes & backup

Definition:

• A database is a set of files stored on disk.
• An instance is a collection of Oracle background processes and shared memory.

Relationship:

• An instance can mount and open one and only one database.
• A database may be mounted and opened by one or more instances.

For example, in the case of Real Application Clusters (or RAC) a database is usually mounted and opened by many instances.

Related documentation: Instance, Database

SELECT comp_name, status, version FROM dba_registry;

To view version numbers of core library components, including the database:

SELECT banner FROM v$version;

To display options that are installed (the value column is ‘FALSE’ if the option is not installed)

SELECT parameter, value FROM v$option;

or

select instance_name, host_name from v$instance;

or

SELECT SYS_CONTEXT(‘USERENV’,’DB_NAME’) FROM DUAL;