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OCA/OCP Oracle Database 11g All-in-One Exam Guide
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If you are using UNDO_MANAGEMENT=AUTO, you must also specify UNDO_ TABLESPACE This parameter nominates a tablespace, which must have been created as an undo tablespace, as the active undo tablespace All the undo segments within it will be brought online (that is, made available for use) automatically Lastly, UNDO_RETENTION, set in seconds, is usually optional It specifies a target for keeping inactive undo data and determines when it becomes classified as expired rather than unexpired If, for example, your longest running query is thirty minutes, you would set this parameter to 1800 Oracle will then attempt to keep all undo data for at least 1800 seconds, and your query should therefore never fail with ORA-1555 If, however, you do not set this parameter, or set it to zero, Oracle will still keep data for as long as it can anyway The algorithm controlling which expired undo data is overwritten first will always choose to overwrite the oldest bit of data; therefore, UNDO_RETENTION is always at the maximum allowed by the size of the tablespace Where the UNDO_RETENTION parameter is not optional is if you have configured guaranteed undo retention The default mode of operation for undo is that Oracle will favor transactions over queries If the sizing of the undo tablespace is such that a choice has to be made between the possibility of a query failing with ORA-1555 and the certainty of a transaction failing with ORA-30036, Oracle will choose to let the transaction continue by overwriting undo data that a query might need In other words, the undo retention is only a target that Oracle will try to achieve But there may be circumstances when successful queries are considered more important than successful transactions An example might be the end-of-month billing run for a utilities company, when it might be acceptable to risk transactions being blocked for a few hours while the reports are generating Another case is if you are making use of flashback queries, which rely on undo data Guaranteed undo retention, meaning that undo data will never be overwritten until the time specified by the undo retention has passed, is enabled at the tablespace level This attribute can be specified at tablespace creation time, or an undo tablespace can be altered later to enable it Once you activate an undo tablespace for which retention guarantee has been specified, all queries will complete successfully, provided they finish within the undo retention time; you will never have snapshot too old errors again The downside is that transactions may fail for lack of undo space If the UNDO_RETENTION parameter has been set, and the datafile(s) making up the undo tablespace is set to autoextend, then Oracle will increase the size of the datafile automatically if necessary to keep to the undo retention target This combination of guaranteed undo retention and autoextending datafiles means that both queries and transactions will always succeed assuming you have enough disk space If you don t, the automatic extension will fail A database might have one tablespace used in normal operations where undo retention is not guaranteed, and another to be used during month-end reporting where retention is guaranteed
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The undo tablespace should be large enough to store the worst case of all the undo generated by concurrent transactions, which will be active undo, plus enough unexpired
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undo to satisfy the longest running query In an advanced environment, you may also have to add space to allow for flashback queries as well The algorithm is simple: calculate the rate at which undo is being generated at your peak workload, and multiply by the length of your longest query The V$UNDOSTAT view will tell you all you need to know There is also an advisor within Database Control that will present the information in an immediately comprehensible way Figure 8-8 shows the undo management screen of Database Control To reach this, take the Server tab from the database home page, then the Automatic Undo Management link in the Database Configuration section The configuration section of the screen shows that the undo tablespace currently in use is called UNDO1, and it is 100MB in size Undo guarantee has not been set, but the datafile(s) for the tablespace is auto-extensible Making your undo datafiles auto-extensible will ensure that transactions will never run out of space, but Oracle will not extend them merely to meet the UNDO_RETENTION target; it is therefore still possible for a query to fail with snapshot too old However, you should not rely on the auto-extend capability; your tablespace should be the correct size to begin with The Change Tablespace button will issue an ALTER SYSTEM command to activate an alternative undo tablespace Further information given on the System Activity tab, shown in Figure 8-9, tells you that the peak rate for undo generation was only 1664KB per minute, and the
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