I am using Automatic Undo Management but still keep on getting ORA-1555 (snapshot too old) error messages. How can I prevent it?
ORA -1555 errors are generated when the undo data required to create a read consistent snapshot is not available. This can happen due to two reasons. 1. The undo_retention parameter, which tells the database instance how long should it preserve a piece of undo data, is set to smaller than the required value. 2. The undo tablespace is not large enough. You may therefore need to adjust the setting of the undo_retention parameter based on the duration of the longest running query. Executing the following query should give you an idea of what this value should be: select max(maxquerylen) from v$undostat; The view v$undostat displays the undo usage information for the last one week and hence, should be able to guide you reasonable well in setting the value of the undo_retention parameter. This must also be backed up making enough space available in the undo tablespace, since the database may be forced to dishonor the undo retention in order to avoid a transaction failure. The EM undo tables
