Sybase 12.4.2 Manuale Utente

Checkpoints, savepoints, and transaction rollback

 

You can undo all changes after a savepoint by issuing a 

ROLLBACK TO 

SAVEPOINT

. This command rolls back to the savepoint you specify, or to the 

most recent 

SAVEPOINT

 if you do not specify a named savepoint. Rolling back 

to savepoint n undoes all actions for all savepoints greater than or equal to n.

Normally, locks are released only at the end of a transaction. However, 

ROLLBACK TO SAVEPOINT

 does release locks under certain conditions, as in 

the following scenario.

Example

Assume that you have a series of savepoints in a transaction, and then perform 
a write operation. You then roll back the transaction to an earlier savepoint. The 
rollback undoes all actions after that savepoint, including the write operation 
and any locks it acquires after the savepoint you are rolling back to. 

IQ sets an implicit savepoint before and after every DML command. The data 
page versions associated with these savepoints are released when the command 
completes. If you want to retain data page versions beyond the end of a single 
DML command, you need to set your own, named savepoints.

Savepoints can be named and they can be nested. By using named, nested 
savepoints, you can have many active savepoints within a transaction. Changes 
between a 

SAVEPOINT 

and a 

RELEASE SAVEPOINT 

can still be canceled by 

rolling back to a previous savepoint or rolling back the transaction itself. 
Changes within a transaction are not a permanent part of the database until the 
transaction is committed. All savepoints are released when a transaction ends.

Savepoints cause Adaptive Server IQ to update information it maintains about 
the location of available disk space. This information is used during transaction 
rollback. 

There is no additional overhead in using savepoints, although unreleased 
savepoints may consume extra disk space by keeping older intermediate 
versions active.