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ORA-01555 Snapshot Too Old


Oracle Tips by Burleson Consulting

The ORA-01555 is caused by Oracle read consistency mechanism.  If you have a long running SQL that starts at 10:30 AM, Oracle ensures that all rows are as they appeared at 10:30 AM, even if the query runs until noon!

Oracles does this by reading the "before image" of changed rows from the online undo segments.  If you have lots of updates, long running SQL and too small UNDO, the ORA-01555 error will appear.

From the docs we see that the ORA-01555 error relates to insufficient undo storage or a too small value for the undo_retention parameter:

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.

You can get an ORA-01555 error with a too-small undo_retention, even with a large undo tables.  However, you can set a super-high value for undo_retention and still get an ORA-01555 error.  Also see these important notes on commit frequency and the ORA-01555 error

The ORA-01555 snapshot too old error can be addressed by several remedies:

  1.  Re-schedule long-running queries when the system has less DML load.

  2. Increasing the size of your rollback segment (undo) size.  The ORA-01555 snapshot too old also relates to your setting for automatic undo retention.

  3. Don't fetch between commits.

Avoiding the ORA-01555 error

Steve Adams has good notes on avoiding the ora-1555 snapshot too old error:

  • Do not run discrete transactions while sensitive queries or transactions are running, unless you are confident that the data sets required are mutually exclusive.

  • Schedule long running queries and transactions out of hours, so that the consistent gets will not need to rollback changes made since the snapshot SCN. This also reduces the work done by the server, and thus improves performance.

  • Code long running processes as a series of restartable steps.

  • Shrink all rollback segments back to their optimal size manually before running a sensitive query or transaction to reduce risk of consistent get rollback failure due to extent deallocation.

  • Use a large optimal value on all rollback segments, to delay extent reuse.

  • Don't fetch across commits. That is, don't fetch on a cursor that was opened prior to the last commit, particularly if the data queried by the cursor is being changed in the current session.

  • Use a large database block size to maximize the number of slots in the rollback segment transaction tables, and thus delay slot reuse.

  • Commit less often in tasks that will run at the same time as the sensitive query, particularly in PL/SQL procedures, to reduce transaction slot reuse.

  • If necessary, add extra rollback segments (undo logs) to make more transaction slots available.  


Oracle ACE Steve Karam also has advice on avoiding the ORA-01555: Snapshot too old, rollback segment too small with UNDO sizing.

Question: I am updating 1 million rows on Oracle 10g, and I run it as batch process, committing after each batch to avoid undo generation. But in Oracle 10g I am told undo management is automatic and I do not need run the update as batch process.

Answer: Automatic undo management was available in 9i as well, and my guess is you were probably using it there. However, I’ll assume for the sake of this writing that you were using manual undo management in 9i and are now on automatic.

Automatic undo management depends upon the  UNDO_RETENTION parameter, which defines how long Oracle should try to keep committed transactions in UNDO segments. However, the UNDO_RETENTION parameter is only a suggestion. You must also have an UNDO tablespace that’s large enough to handle the amount of UNDO you will be generating/holding, or you will get "ORA-01555: Snapshot too old, rollback segment too small" errors.

You can use the UNDO advisor to find out how large this tablespace should be given a desired UNDO retention, or look online for some scripts…just Google for: oracle undo size

Oracle 10g also gives you the ability to guarantee undo. This means that instead of throwing an error on SELECT statements, it guarantees your UNDO retention for consistent reads and instead errors your DML that would cause UNDO to be overwritten.

Now, for your original question…yes, it’s easier for the DBA to minimize the issues of UNDO when using automatic undo management. If you set the UNDO_RETENTION high enough with a properly sized undo tablespace you shouldn’t have as many issues with UNDO.

How often you commit should have nothing to do with it, as long as your DBA has properly set UNDO_RETENTION and has an optimally sized UNDO tablespace. Committing more often will only result in your script taking longer, more LGWR/DBWR issues, and the “where was I” problem if there is an error (if it errors, where did it stop?).

Lastly (and true even for manual undo management), if you commit more frequently, you make it more possible for ORA-01555 errors to occur. Because your work will be scattered among more undo segments, you increase the chance that a single one may be overwritten if necessary, thus causing an ORA-01555 error for those that require it for read consistency.

It all boils down to the size of the undo tablespace and the undo retention, in the end…just as manual management boiled down to the size, amount, and usage of rollback segments. Committing frequently is a peroxide band-aid: it covers up the problem, tries to clean it, but in the end it just hurts and causes problems for otherwise healthy processes.


Oracle guru Joel Garry offers another great explanation of  the machinations of the ORA-01555 error:

You have to understand, in general, ORA-01555 means something else is causing it to die - Oracle needs to be able to create a read-consistent view of the table for the query as it looked at the start of the query, and it is unable to because something has overwritten the undo necessary to create such a view. Since you have the same table over and over in your alert log, that probably means the something is the previous queries your monitoring software is making, not ever releasing the transaction.

Something like:

  1. 10AM query starts, never ends

  2. 11AM query starts, never ends

  3. Noon query starts, never ends

  4. 1PM query starts

Meanwhile, the undo needed from the 10AM query for the 1PM query gets overwritten, 1PM query dies with ORA-01555, since it needs to know what the table looked like before the 10AM query started mucking with it.

Also if the query is a loop with a commit in it, it can do the same thing without other queries, as eventually the next iteration requires looking back at it's own previous first generation, can't do it, and barfs.

Upping undo_retention may help, or may not, depending on the real cause. Also check v$undostat, you may still have information in there if this is ongoing (or may not, since by the time you check it the needed info may be gone). 


Also see our notes on ORA-01555 here:

 
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