What is SID in Oracle ?

In the doc you’ll find, it’s the Oracle system identifier.

Okay, let’s imagine the following

Environment:

ORACLE_SID=ORA001

init.ora:

DB_NAME=DB001
DB_UNIQUE_NAME=UNI001
INSTANCE_NAME=INS001
SERVICE_NAMES=SVC001,SVC002
DB_DOMAIN=EXAMPLE.COM
GLOBAL_NAMES=false

database:

SQL> select * from GLOBAL_NAME;
GLO001.example.com

listener.ora:

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (GLOBAL_NAME=GLO001.EXAMPLE.COM)
      (SID_NAME=ORA001)
    )
  )

What is my SID? Actually there is more than one correct answer.

In the environment, Oracle SID is ORA001. This matches SID_NAME in listener.ora. It does not have to match database name, unique name, global name or instance_name.

$ lsnrctl services
Services Summary...
Service "GLO001.EXAMPLE.COM" has 1 instance(s).
  Instance "ORA001", status UNKNOWN, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:7 refused:0
         LOCAL SERVER

As the instance is not running, I have only my listener.ora static connections.

The SERVICE_NAME is GLO001.EXAMPLE.COM and the SID is ORA001.

$ sqlplus "sys/***@
  (DESCRIPTION=
    (ADDRESS=
      (PROTOCOL=TCP)
      (HOST=SRV001)
      (PORT=1521)
    )
    (CONNECT_DATA=
      (SERVICE_NAME=GLO001.EXAMPLE.COM)
    )
  )" as sysdba
Connected to an idle instance.
SQL> 
$ sqlplus "sys/***@
  (DESCRIPTION=
    (ADDRESS=
      (PROTOCOL=TCP)
      (HOST=SRV001)
      (PORT=1521)
    )
    (CONNECT_DATA=
      (SID=ORA001)
    )
  )" as sysdba
Connected to an idle instance.
SQL> 

Let’s start

SQL> startup

and check my services

$ lsnrctl services
Services Summary...
Service "SVC001.EXAMPLE.COM" has 1 instance(s).
  Instance "INS001", status READY, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:0 refused:0 state:ready
         LOCAL SERVER
Service "SVC002.EXAMPLE.COM" has 1 instance(s).
  Instance "INS001", status READY, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:0 refused:0 state:ready
         LOCAL SERVER
Service "UNI001.EXAMPLE.COM" has 1 instance(s).
  Instance "INS001", status READY, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:0 refused:0 state:ready
         LOCAL SERVER
Service "GLO001.EXAMPLE.COM" has 1 instance(s).
  Instance "ORA001", status UNKNOWN, has 1 handler(s) for this service...
    Handler(s):
      "DEDICATED" established:13 refused:0
         LOCAL SERVER
The command completed successfully

I know have 4 service names :

1. The global name in listener.ora
2. the unique name in init.ora
3. both service name in init.ora

And 2 sid

1. The SID in listener.ora
2. The instance name in init.ora

While we often have sid = oracle_sid = service_name = service_names = global_name = instance_name = db_name = db_unique_name, if you switch from SID to SERVICE_NAME, this could be help to identify legacy application.

If you read the doc carefully, you may have noticed the SID is no longer documented as a valid clause of CONNECT_DATA in 11g and 12c

In 10gR2 :
http://docs.oracle.com/cd/B19306_01/network.102/b14213/tnsnames.htm#i477921
Use the parameter SID to identify the Oracle8 database instance by its Oracle System Identifier (SID). If the database is Oracle9i or Oracle8, use the SERVICE_NAME parameter rather than the SID parameter.

This is probably a documentation bug, I would rephrase this as If the database is Oracle9i or Oracle8i or later.

In 11g and 12c, the comment disappeared. Oracle 8i was released last century, but SID didn’t completly disappear from tnsnames. Yet.