javascript barcode scanner Transactions and Undo Data in Software

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When a transaction starts, Oracle will assign it to one (and only one) undo segment Any one transaction can only be protected by one undo segment it is not possible for the undo data generated by one transaction to cut across multiple undo segments This is not a problem, because undo segments are not of a fixed size So if a transaction does manage to fill its undo segment, Oracle will automatically add another extent to the segment, so that the transaction can continue It is possible for multiple transactions to share one undo segment, but in normal running this should not occur A tuning problem common with rollback segments was estimating how many rollback segments would be needed to avoid excessive interleaving of transactions within rollback segments without creating so many as to waste space One feature of undo management is that Oracle will automatically spawn new undo segments on demand, in an attempt to ensure that it is never necessary for transactions to share undo segments If Oracle has found it necessary to extend its undo segments or to generate additional segments, when the workload drops Oracle will shrink and drop the segments, again automatically
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EXAM TIP No transaction can ever span multiple undo segments, but one undo segment can support multiple transactions As a transaction updates table or index data blocks, the information needed to roll back the changes is written out to blocks of the assigned undo segment All this happens in the database buffer cache Oracle guarantees absolutely the A, for atomicity, of the ACID test, meaning that all the undo data must be retained until a transaction commits If necessary, the DBWn will write the changed blocks of undo data to the undo segment in the datafiles By default, Oracle does not, however, guarantee the C, for consistency, of the ACID test Oracle guarantees consistency to the extent that if a query succeeds, the results will be consistent with the state of the database at the time the query started but it does not guarantee that the query will actually succeed This means that undo data can be categorized as having different levels of necessity Active undo is undo data that might be needed to roll back transactions in progress This data can never be overwritten, until the transaction completes At the other extreme, expired undo is undo data from committed transactions, which Oracle is no longer obliged to store This data can be overwritten if Oracle needs the space for another active transaction Unexpired undo is an intermediate category; it is neither active nor expired: the transaction has committed, but the undo data might be needed for consistent reads, if there are any long-running queries in progress Oracle will attempt not to overwrite unexpired undo EXAM TIP Active undo can never be overwritten; expired undo can be overwritten Unexpired undo can be overwritten, but only if there is a shortage of undo space The fact that undo information becomes inactive on commit means that the extents of undo segments can be used in a circular fashion Eventually, the whole of the undo tablespace will be filled with undo data, so when a new transaction starts, or a running transaction generates some more undo, the undo segment will wrap around, and the oldest undo data within it will be overwritten always assuming that this oldest data is not part of a long-running uncommitted transaction, in which case it would be necessary to extend the undo segment instead With the old manually managed rollback segments, a critical part of tuning was to control which transactions were protected by which rollback segments A rollback segment might even be created and brought online specifically for one large transaction Automatically managed undo segments make all of that unnecessary, because you as DBA have no control over which undo segment will protect any one transaction Don t worry about this Oracle does a better job that you ever could But if you wish you can still find out which segment has been assigned to each transaction by querying the view V$TRANSACTION, which has join columns to V$SESSION and DBA_ROLLBACK_SEGS, thus letting you build up a complete picture of transaction activity in your database: how many transactions there are currently running, who is running them, which undo segments are protecting those transactions, when the
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