Question on "nested txns" vs "autonomous txns"]

For people who have requirements to code a Autonomous txn in Oracle which is called in general as a "Nested Top level (sub)transaction",
Things to watchout are esp in the context of temp tables,
Autonomous transactions just happen in another transaction but within the same session of parent/main txn and so your temp tables/gtt s created in main txn should be accessible from within autonomous txn except for the uncommitted changes done to it in main txn immediately (Infact if you cannot even populate a same gtt/temp table which has been already populated in main txn, you would get error in autonomous txn )

I did some study and found that use of autonmous txn is 99% for auditing/logging purposes and as Thomas Kyte(asktom) says any other use of them is sure a problem in design/code.You would need to really check your logic for such a use case before you decide some txn as a autonomous txn.


I would summarise the following for autonomous and nested txns:

Autonomous Txn:

• Autonomous txn is just a different independent transaction from the main/parent txn but within the same session . Hence does not share transactional resources as that of main/parent txn.
• Cannot see uncommitted changes in main/parent txn for consistency reasons. (Note: Consistency is always at transactional level and not at session level)
• After a commit in autonomous txn you return immediately to the transactional context of main/parent transaction.i.e after a commit in autonomous txn you are back to parent txn.
  
• Autonomous txn since they operate within the same session they can access the gtt/temp tables but cannot see data in them already populated by main/parent txn. Infact they get error when they try to populate them if done already in parent txn. But they can populate them if not already populated in parent/main txn which would be a rare case.
• Changes made in autonomous txn are visible to parent/main txn based on isolation level set by main/parent transaction using the "set transaction isolation level .." statement in pl/sql. Oracle,by default makes committed changes visible(i,e "read committed" isolation level is default for any DML statement level) but "serializable" isolation level can be set for a transaction level using "set transaction isolation level serializable" statement for multi-statement read consistency. i.e the main/parent transaction would not see any committed changes made later by other transactions including autonomous txn which is also a different txn. Hence if your parent/main txn starts with a set transaction ..serializable it wont see any committed changes done in the autonomous child txn of it. Normally you would use this "serializable" isolation level for a short time OLTP txns and most often go with default "read committed" statement level.
• Exceptions raised from within autonomous txn get rolled back to transaction level and not to statement level.
  
• Use of Autonomous txn in XA/distributed env was not supported in 9i and not sure of complications in later releases.
  
• Autonomous txn use cases are very rare and 99.99% they are for logging/auditing purposes.

Nested Txn:

• For Oracle nested txn simple mean a transaction done from within a parent/main txn as i explained already.You can only set savepoints and rollback to them as you know already. You can use JDBC 3.0 standard savepoints interface for this in java or use pl/sql savepoints to rollback incrementally as you said.
• Transaction isolation levels can also be set using jdbc APIs. Oracle as said above only support default "read committed" and "serialization" levels.
  
• In Oracle Nested txns always see uncommitted changes in parent/main txn and changes in nested child txns are also always visible for parent txn.
  

Please see autonomous and nested transaction example.

Autonomous txn Example (In PL/SQL)
create table audit_test
(
name varchar2(20),
join_date date,
identifier varchar2(200),
log_id number
)
/
Truncate table audit_test
/
Create or replace procedure commit_test
is
pragma autonomous_transaction ;
v_nr number;
begin
select nvl(max(log_id),0) into v_nr from audit_test;
dbms_output.put_line('Maximum before autonomous txn In Child:'||v_nr); --(0)Autonomous Child txn wont see uncommitted changes in Parent.
insert into audit_test values('laksA',sysdate-1,'TestA',2);
commit; --Autonomous Child txn has to a commit/rollback always
end;
/
declare
v_nr number ;
begin
set transaction isolation level serializable name 'Parent'; -- named 'Parent' txn
-- This main/parent transaction sees db as of this time for multi -statement consistency
-- Without "serialization" isolation level this main txn would see the committed changes of autonomous child txn below
insert into audit_test values('laks',sysdate,'Test',1) ;
commit_test; -- calls autonomous child txn
select max(log_id) into v_nr from audit_test ;
dbms_output.put_line('Maximum after autonomous txn In Parent : '|| v_nr); --Output should be 1 with "serializable" and 2 without it in parent txn.
rollback ; -- Doesnt affect committed changes of the autonomous child transaction.
end;
/

Nested Transaction eg (In PL/SQL_)
create table audit_test
(
name varchar2(20),
join_date date,
identifier varchar2(200),
log_id number
)
/
Truncate table audit_test
/
Create or replace procedure commit_test
is
v_nr number;
begin
-- set transaction name 'Child' ; You cannot start a true nested txn like this.
select nvl(max(log_id),0) into v_nr from audit_test ;
dbms_output.put_line('Maximum before child txn in Child:'||v_nr); --(1) Nested Child txn always sees uncommitted changes in Parent txn
insert into audit_test values('laksA',sysdate-1,'TestA',2);
commit; --Everything done in Child as well as in Parent prior to callign Child gets committed.
end;
/
declare
v_nr number ;
begin
set transaction isolation level serializable name 'Parent'; -- named 'Parent' txn
-- This main/parent transaction sees db as of this time for multi -statement consistency
-- Parent would always sees uncommitted/committed changes in nested child txn.
insert into audit_test values('laks',sysdate,'Test',1) ;
commit_test; -- calls nested child txn named 'Child'
select max(log_id) into v_nr from audit_test ;
dbms_output.put_line('Maximum after Child Txn in Parent '|| v_nr); --Output should be 2 in parent txn always.
rollback ; --No Use at all .
end;
/

Looks like there is no true nested transaction API support in Oracle.

All these transaction concepts esp isolation level for a transaction implemented by DBMS vendors are requirements from TPC(Transaction Processing performance council) which sets few standards and req, to publish benchmark results.
JTA/JTS (java transaction API/java transaction service) is also a driving force for Transaction API provided by vendors. Oracle implementation of transaction API is much different from other DB vendors and also for performance/integrity reasons Oracle doesnt provide some features.