What is snapshot too old error and how do you fix it?

ORA-01555 Error Message “Snapshot Too Old” Error ORA-01555 contains the message, “snapshot too old.” This message appears as a result of an Oracle read consistency mechanism. While your query begins to run, the data may be simultaneously changed by other people accessing the data.

What is Ora-01555 snapshot too old?

ORA-01555: snapshot too old: rollback segment number string with name “string” too small. Cause: Rollback records needed by a reader for consistent read are overwritten by other writers. Action: If in Automatic Undo Management mode, increase the setting of UNDO_RETENTION. Otherwise, use larger rollback segments.

How do you resolve ORA-01555 caused by SQL statement?

To resolve the issue, increase the size of your rollback segment (undo) size.

What is snapshot in Oracle with example?

A snapshot refresh is an efficient batch operation that makes a snapshot reflect a more current state of its master table. You must decide how and when to refresh each snapshot to make it more current. For example, snapshots based on master tables that applications update often may require frequent refreshes.

What is undo retention period in Oracle?

When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information before overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired.

How do you increase undo retention?

Do one of the following:

  1. Set UNDO_RETENTION in the initialization parameter file. UNDO_RETENTION = 1800.
  2. Change UNDO_RETENTION at any time using the ALTER SYSTEM statement: ALTER SYSTEM SET UNDO_RETENTION = 2400;

How do I calculate the size of my undo tablespace?

The following formula calculates the peak undo blocks generated per second: SQL> SELECT undoblks/((end_time-begin_time)*86400) “Peak Undo Block Generation” FROM v$undostat WHERE undoblks=(SELECT MAX(undoblks) FROM v$undostat);

How do I change the snapshot isolation level?

You need to enable snapshot isolation by setting the ALLOW_SNAPSHOT_ISOLATION database option in order to use it. The READ_COMMITTED_SNAPSHOT database option determines the behavior of the default READ COMMITTED isolation level when snapshot isolation is enabled in a database.

Can we create snapshot for another schema?

To create a snapshot in another user’s schema, you must have the CREATE ANY SNAPSHOT system privilege, as well as SELECT privilege on the master table. Additionally, the owner of the snapshot must have been able to create the snapshot.

How do I change undo retention period?

Setting the Undo Retention Period

  1. Set UNDO_RETENTION in the initialization parameter file. UNDO_RETENTION = 1800.
  2. Change UNDO_RETENTION at any time using the ALTER SYSTEM statement: ALTER SYSTEM SET UNDO_RETENTION = 2400;

What happens when undo tablespace is full?

The UNDO tablespace works as follows: * The UNDO records are not deleted when they are expired. They stay and are overwritten only when a new UNDO record needs to be written and there is no free space. Thus, it is normal for UNDOTBS1 to appear at 99% full.

How do you check datafiles in undo tablespace?

View Undo storage.

  1. Use the following queries to view undo tablespaces and rollback segments: SELECT * FROM DBA_TABLESPACES WHERE CONTENTS = ‘UNDO’; SELECT * FROM V$PARAMETER WHERE NAME = ‘undo_tablespace’;
  2. Use the following queries to view undo statistics: SELECT * FROM V$ROLLSTAT; SELECT * FROM V$UNDOSTAT;