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As discussed in 11, the Undo Advisor will observe the rate of undo data generation and the length of queries being run, and it will recommend a minimum size for the undo tablespace, which will ensure that queries do not fail with a snapshot too old error
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The mechanism for instance recovery after a failure is detailed in 15 In summary, if the instance terminates in a disorderly fashion (such as a power cut or server reboot while the database was open, or just a SHUTDOWN ABORT), then on the next startup it is necessary to reinstate all work in progress that had not been written to the datafiles at the time of the crash This will happen automatically, but until it is done, users cannot log on The MTTR advisor estimates how long this period of downtime for crash recovery will be, given the current workload
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If the database has been damaged in some way (such as files deleted or data blocks corrupted), it may take some time to identify the problem Then there will often be several way of recovering from the situation For example, if a number of datafiles have been damaged by corruptions appearing on a disk, it will be necessary to find out which files, and which blocks Then a decision must be made as to whether to
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restore entire files, or only the damaged blocks If the database is protected by a physical standby, switching over to that would also be a possibility Following a failure, any DBA (no matter how experienced) will need time to determine the nature and extent of the problem, and then more time to decide upon the course of action that will repair the damage with the minimum disruption to work The Data Recovery Advisor follows an expert system to advise the DBA on this The expert system is essentially what the DBA would follow anyway, but the advisor can do it much faster
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Segments grow automatically As rows are inserted into table segments and index keys are inserted into index segments, the segments fill and then Oracle will allocate more extents as necessary But segments do not shrink automatically as data is removed or modified with DELETE and UPDATE commands; this only happens when the segment is deliberately reorganized The segment advisor observes tables and indexes, both their current state and their historical patterns of use, and recommends appropriate reorganization when necessary
Automatic Maintenance Jobs
If the database is to run well, it is vital that the optimizer has access to accurate object statistics; that the tables and indexes are operating efficiently, without a large amount of wasted space and fragmentation; and that the high-load SQL statements have been tuned The gathering of statistics and the running of the Segment Advisor and the SQL Tuning Advisor are, by default, automatic in an 11g database A facility introduced with release 10g of the database was the Scheduler The Scheduler runs jobs automatically, at certain times and repeatedly at certain intervals The jobs will usually be defined by the DBA, but following creation of a database with the DBCA, there will be three tasks configured within what is called the AutoTask system These AutoTasks are
Gathering optimizer statistics Running the Segment Advisor Running the SQL Advisor
The AutoTasks run within the Scheduler s maintenance window A Scheduler window is a repeating time period The maintenance windows are defined to open at 2200 every weeknight and to close four hours later, at 0200, and on Saturday and
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Sunday to open at 0600, closing twenty hours later, at 0200 The Scheduler is linked to another database facility: the Resource Manager The Resource Manager allocates resources to different database sessions The Resource Manager plan that is activated during the maintenance window is one that ensures that no more than 25 percent of machine resources are dedicated to the AutoTask jobs, meaning that running these tasks should not impact adversely on other work If the maintenance window time frames are not suitable for your database workload, they are adjustable; if the maximum of 25 percent resource usage is too high and causes performance for other work to degrade, this too can be changed The underlying assumption is that the AutoTasks will run at a time and with a priority that is unlikely to cause problems for regular work for end users
There are three automated maintenance tasks: gathering optimizer statistics, the Segment Advisor, the SQL Tuning AdvisorThe advisors run automatically, but the recommendations
must be accepted (or ignored) manually The tasks run in the maintenance window, which by default opens for four hours every weeknight at 2200 and for twenty hours on Saturday and Sunday, opening at 0600
For any of the AutoTasks to run, the STATISTICS_LEVEL parameter must be set to TYPICAL (the default) or ALL The simplest way to manage the scheduling of the AutoTask jobs is through Database Control Take the Server tab from the Database Home page, then the Automated Maintenance Tasks link on the Scheduler section The Automated Maintenance Tasks window, shown in Figure 13-8, displays the window for the next run (which opens at 0600, because the figure was taken on a Saturday evening) with links for the results of the last two advisor runs Taking these links will show the recommendations (if any) with an option to implement them (implementing the recommendations is not automatic) The segment advisor task relies on the history of object statistics built up by the daily running of the gather optimizer statistics task By observing these, the Segment Advisor can see not only how much unused space there is in the tables and indexes, but also whether this space is likely to be needed again, and if not advise that the segments should be reorganized The SQL Tuning Advisor task relies on the AWR statistics gathered by the MMON process These statistics include figures on which SQL statements are being execute many times, perhaps millions of times an hour;
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