Re: [PATCHES] Nested transactions

[Barry Lind]

Alvaro Herrera wrote:
With this in place, implementing SAVEPOINTs the way SQL expects them to
work appears to be a very trivial exercise.
You may not see it, but a savepoint is just the start of a nested
transaction in disguise.  Consider:
begin;
        insert into foo values (1);
        savepoint dammit;
        insert into foo values (2);
        select foo;             -- fails
        rollback to dammit;
        insert into foo values (3);
commit;
You expect the transaction to finish with tuples 1 and 3 in table foo,
right?  Well, this is exactly the same as
begin;
        insert into foo values (1);
        begin;          -- dammit
                insert into foo values (2);
                select foo;     -- fails, goes to aborted state
        rollback;
        insert into foo values (3);
commit;
So all that's needed for the former to work is to be able to define a
"name" for a transaction (using a cute syntax) and being able to
rollback to it.  Definitely trivial, after all the work I have put into
making the latter work.
The problem I see with moving towards supporting savepoints with the 
current proposal is with how commit works:

Consider:
begin;
        insert into foo values (1);
        savepoint dammit;
        insert into foo values (2);
        select foo;
        insert into foo values (3);
commit;
This one commit needs to commit the top level transaction.  But if the 
savepoint command is really starting a sub transaction then that commit 
would only commit the subtransaction not the top level transaction.  I 
don't see how you can use COMMIT to sometimes mean commit the 
subtransaction and other times have it mean commit the top level 
transaction.

I don't have a problem with the under the covers functionality in this 
patch, it is how begin/commit are changed to support the underlying 
functionality that concerns me.  IMHO we should not change the behavior 
of begin/commit for nested transactions (leave them do what they have 
always done - i.e. control the top level transaction state), but 
introduce new syntax for subtransactions control.

thanks,
--Barry

---------------------------(end of broadcast)---------------------------
TIP 2: you can get off all lists at once with the unregister command
   (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])