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Flashback Query

Flashback cursors

Flashback query allows to get data as of required point in time. It’s a nice feature. It’s strange in the implementation though. If you try to use AS OF SCN query with bind variables, it won’t help you to keep number of child cursors low: each execution (independently of the incoming SCN value) will cause a new child cursor to appear with FLASHBACK_CURSOR as the cause. I don’t understand why Oracle is doing it this way.

Flashback Query "AS OF" - Tablescan costs

This is just a short note prompted by a recent thread on the OTN forums. In recent versions Oracle changes the costs of a full table scan (FTS or index fast full scan / IFFS) quite dramatically if the "flashback query" clause gets used.

It looks like that it simply uses the number of blocks of the segment as I/O cost for the FTS operation, quite similar to setting the "db_file_multiblock_read_count" ("dbfmbrc"), or from 10g on more precisely the "_db_file_optimizer_read_count", to 1 (but be aware of the MBRC setting of WORKLOAD System Statistics, see comments below) for the cost estimate of the segment in question.

This can lead to some silly plans depending on the available other access paths as can be seen from the thread mentioned.

Life Cycle of a Process Request

Oracle's Flashback Query facility lets you query a past version of a row by using the information in the undo segment.  The VERSIONS option lets you seen all the versions that are available. Thus, it is possible to write a simple query to retrieve the all values that changed on a process request record through its life cycle.

The Oracle parameter undo_retention determines how long that data remains in the undo segment. In my example, it is set to 900 seconds, so I can only query versions in the last 15 minutes. If I attempt to go back further than this I will get an error.

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column prcsinstance heading 'P.I.' format 9999999

Life Cycle of a Process Request

Oracle's Flashback Query facility lets you query a past version of a row by using the information in the undo segment.  The VERSIONS option lets you seen all the versions that are available. Thus, it is possible to write a simple query to retrieve the all values that changed on a process request record through its life cycle.

The Oracle parameter undo_retention determines how long that data remains in the undo segment. In my example, it is set to 900 seconds, so I can only query versions in the last 15 minutes. If I attempt to go back further than this I will get an error.

#eeeeee; border: 0px solid rgb(0, 0, 0); overflow: auto; padding-left: 4px; padding-right: 4px; width: 100%;">
column prcsinstance heading 'P.I.' format 9999999