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The FLASHBACK DATABASE command was introduced to speed up the otherwise slow process of point-intime database recovery. It can be used in place of a full database restore and a rolling forward using archive logs, and it is primarily designed to speed up the recovery from an accident. For example, let s take a look at what a DBA might do to recover from an accidentally dropped schema, in which the right schema was dropped, just in the wrong database (it was meant to be dropped in the test environment). The DBA immediately recognizes the mistake he has made and shuts down the database right away. Now what Prior to the FLASHBACK DATABASE capability, what would probably happen is this: 1. 2. 3. 4. 5. The DBA would shut down the database. The DBA would restore the last full backup of database from tape (typically), generally a long process. The DBA would restore all archive redo logs generated since the backup that were not available on the system. The DBA would roll the database forward and stop rolling forward at a point in time just before the erroneous DROP USER command. The database would be opened with the RESETLOGS option.

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This relies on the following logical rules for constructing BDD nodes formed by taking the conjunction of existing nodes Note how the second rule is used to interleave variables (x => P | Q) AND (x => R | S) is identical to (x => P AND R | Q AND S) (x => P | Q) AND (y => R | S) is identical to (x => P AND T | Q AND T) where T is simply (y => R | S) One final important optimization in the implementation is to memoize the application of the mkAnd operation Given the previous implementation of BDDs, you can now add the members ToString to convert BDDs to strings, Build to convert a Prop representation of a formula into a BDD, and Equiv to check for equivalence between two BDDs: member g.

This was a nontrivial process with many steps and would generally consume a large piece of time (time when no one could access the database, of course). The causes of a point-in-time recovery like this are many: an upgrade script gone awry, an upgrade gone bad, an inadvertent command issued by someone with the privilege to issue it (a mistake, probably the most frequent cause), or some process introducing data integrity issues into a large database (again, an accident; maybe it was run twice instead of just once, or maybe it had a bug). Whatever the reason, the net effect was a large period of downtime. The steps to recover in Oracle 10g Enterprise Edition and above, assuming you configured the flashback database capability, would be as follows: 1. 2. The DBA shuts down the database. The DBA startup-mounts the database and issues the flashback database command, using either an SCN, the Oracle internal clock, or a timestamp (wall clock time), which would be accurate to within a couple of seconds. The DBA opens the database with resetlogs.

ToString(Bdd idx) = let rec fmt depth idx = if depth > 3 then ".." else let (Node(p,l,r)) = idxToNode(idx) if p = "" then if l = trueIdx then "T" else "F" else sprintf "(%s => %s | %s)" p (fmt (depth+1) l) (fmt (depth+1) r) fmt 1 idx.

To use this feature, the database must be in ARCHIVELOG mode and must have been set up to enable the FLASHBACK DATABASE command. What I m trying to say is that you need to set up this capability before you ever need to use it. It is not something you can enable after the damage is done; you must make a conscious decision to use it, whether you have it on continuously or whether you use it to set restore points.