Questions about Rollback - after insert, update, delete ... operations?

[Emi Lu <emilu@xxxxxxxxxxxxxxx>]

greetings,

I remembered I read something in the mailing list about "*rollback*" a 
while ago. People mentioned that some operations cannot rollback.
I cannot remember what kinds of perations are not be able to rollback?

For example,

begin
   ... ...
   insert
   ... ...
   delete
   ... ...
   update
   ... ...
  
   /* If any of the above operation failed, we can rollback all the 
above operations? */
   rollback
   ... ...
end


Will all "Insert, delete, update" operations rollback if any of the 
operations fails?

Thanks a lot!
Emi

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