What is the difference between USER() and SYS_CONTEXT('USERENV','CURRENT_USER')?

Question

In an Oracle Database, what are the differences between the following:

• user()
• sys_context(\'USERENV\', \'CURRENT_USER\')
• sys_context(\'USEREN

Answer 1

sys_context('USERENV', 'CURRENT_SCHEMA') - The schema that is currently being used and as you already found out can be changed with alter session

sys_context('USERENV', 'SESSION_USER') - The user that was used for authentication during the creation of the session and cannot be changed

sys_context('USERENV', 'CURRENT_USER') - Pretty much like "session_user" deprecated (at least according to the 10g documentation)
(editted according to @a_horse_with_no_name's answer and the reference he gave to the 11g docs)

sys_context('USERENV', 'AUTHENTICATED_IDENTITY') - The identity used for authentication, depends on the "AUTHENTICATION_METHOD".
from documentation:

• Kerberos-authenticated enterprise user: kerberos principal name
• Kerberos-authenticated external user : kerberos principal name; same as the schema name
• SSL-authenticated enterprise user: the DN in the user's PKI certificate
• SSL-authenticated external user: the DN in the user's PKI certificate
• Password-authenticated enterprise user: nickname; same as the login name
• Password-authenticated database user: the database username; same as the schema name
• OS-authenticated external user: the external operating system user name
• Radius/DCE-authenticated external user: the schema name
• Proxy with DN : Oracle Internet Directory DN of the client
• Proxy with certificate: certificate DN of the client
• Proxy with username: database user name if client is a local database user; nickname if client is an enterprise user.
• SYSDBA/SYSOPER using Password File: login name
• SYSDBA/SYSOPER using OS authentication: operating system user name

user pseudo column - I'm not sure, according to documentation I'd think it's like CURRENT_SCHEMA but apparently it's like CURRENT_USER