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The MAXQUERYLEN column of the V$UNDOSTAT view tells you the length of the longest executed query (in seconds) during the past 24 hours The time set in the UNDO_RETENTION parameter should be at least as long as the time indicated in the MAXQUERYLEN column This, by itself, won t guarantee that a new long-running query will definitely be accommodated, but you ll have a reasonable chance that your longest transactions will have read consistency when using the undo tablespace Oracle provides the following guidelines for setting the undo retention interval for a new database: OLTP: 15 minutes Mixed: 1 hour DSS: 3 hours Flashback Query: 24 hours If you think all of this undo retention business is too much work, take the easy way out and let Oracle automatically tune undo in your database.

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Oracle automatically tunes the undo retention period for the longest-running query and collects query-duration information every 30 seconds Depending on your workload characteristics, Oracle will adjust the length of the undo retention period For example, during the day, shorter transactions may mean a shorter undo retention period, and during the nightly batch jobs, you d need a much longer undo retention period to avoid the snapshot-too-old errors If you don t set a value for the UNDO_RETENTION parameter (or if you set a value of 0), Oracle automatically tunes undo with 900 seconds as the minimum value Here s a summary of automatic undo retention in Oracle Database 10g Release 2: If you use an auto-extensible undo tablespace (using an AUTOEXTEND data file), Oracle will treat any UNDO_RETENTION value you specify as the low threshold value and retain undo for at least this time period.

If you set an undo retention period of 30 minutes, Oracle will adjust the retention period upward of 30 minutes if needed, but never let it go below 30 minutes (unless faced with a lack of space in the undo tablespace) The database will tune the undo retention to take care of the undo needs of the longest queries in your database Thus, in the case of auto-extensible undo tablespaces, Oracle will Retain undo data a little longer than the longest query in your database, if space allows it Retain undo data at least as long as the low threshold of undo retention, subject to space limitations If you use a fixed-size undo tablespace, Oracle will ignore any UNDO_RETENTION value you may have set The database will automatically tune undo with the goal of achieving the maximum possible retention period, given the undo tablespace size and its usage history.

Of course, if you use the GUARANTEED RETENTION feature, as explained later in this chapter, Oracle will have to honor any UNDO_RETENTION period you set If you ve specified any Flashback requirements, Oracle will satisfy them as well If you re considering a fixed size and an auto-extensible tablespace of the same size, know that the fixed-size tablespace will provide you with a slightly longer undo retention period Even if you do set a value for the UNDO_RETENTION parameter, Oracle will still auto-tune undo, with the value you specified as the minimum value Note that the value you assign for the UNDO_RETENTION parameter is treated by Oracle as a requested minimum If Oracle determines, through its automatic tuning, that the undo retention period should be longer than this requested minimum to accommodate long transactions, it will retain undo data for the longer retention period..

System Report for casper on Thu Aug 4 21:33:19 PDT 2005 Hostname: casper NIS Domain: (none) Processor: Intel(R) Pentium(R) 4 CPU 1.80GHz Running at 3555.32 bogomips with 256 KB cache OS Type: Linux Kernel: 2.6.10-gentoo-r7 Kernel Compile #2 on Thu Feb 24 14:38:44 PST 2005 Uptime: 163.70 days Memory: 514836 kB Free: 183744 kB %Used: 65 Swap: 506036 kB Free: 491324 kB %Used: 3 Load Average: 0.04 0.11 0.05 Process Count: 140 total 1 running 139 sleeping 0 stopped 0 zombie

DB_KEEP_CACHE_SIZE = 48MB DB_RECYCLE_CACHE_SIZE = 24MB DB_CACHE_SIZE = 128MB /* standard 4KB block size */ DB_2k_CACHE_SIZE =48MB /* 2KB non-standard block size */ DB_8k_CACHE_SIZE =192MB /* 8KB non-standard block size */ DB_16k_CACHE_SIZE = 384MB /* 16KB non-standard block size */ The total buffer cache size in this example will be the sum of all the above subcaches, which comes to about 824MB.