Many time people come accross Snapshot too Old error. there are many reasons for this error but the most basic and important one is that the rollback segments are too small.

Accordingly this error can arise when a long-running query is being executed at the same time as data manipulation transactions.

Possible Remedies:

• Increase the size of all the roll back segments.

• Add more rollback segments.

• Increase the OPTIMAL size of the roll back segments i.e.:

Alter rollback segment XXX storage (optimal NNNK);

• If the error seems to be associated with a particular application (job or script and not necessarily the one producing the error) then investigate whether that application s use of rollback space could be reduced. For example consider whether the application could 'commit' more frequently. For long running applications (particularly if it is the application that is encountering the error) consider whether any record sets could be refreshed periodically rather than relying upon the same record set contents for the entire run of the application.

hi

The snap shot too old can also arise because of LOB.

If we do concurrant writes to same segment then ORACLE will alllow it.

But the data will be corrupted. The subsequent reads will give rise to this error

Yes as U said

If on one side a query is executed and simultaneously a DML is happening at the same time. THEN the user doing the DML action gives a commit in between while the query is still executing on the other side then on the query side U get this message SNAPSHOT TOO OLD

Suggestion :

Alter system set UNDO_retention 900 ;

or give a large value for undo retention ie. the undo block will be kept in the memory until 900 microseconds even after the commit transaction on the same table so that the query ends within that.

Just in simple words.....

Ora01555 error occurs when a transation fails when it cannot able to find read consistence image in rollback segment.

To solve this problem: Increse the Undo_Retention

Cheers :)