Rollback statement Oracle

Question

I'm new to dba in oracle. Im just learning about rollback segment. I have a few questions about it, it'll be great if i can find some help here.

1- after issuing a dml statement, does the most valid data exists in redo log files or rollback statement?

2- is the rollback statement ever used in a ddl statement?

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Answer 1

You can (loosely) think of redo and undo as instructions.

For example, you drove from home to the shop. That is a change in your "state", ie, 20 mins ago you were at home, now you are at the shop.

The redo information would be:

• grab the keys
• start the car
• drive down Main st
• turn left into North ave
• turn right into parking lot
• park car

It is a list of instructions that would let you (aka the database) repeat a task (for example, during a database recovery).

The undo information is not about remembering your previous state (you were at home), but more a set of instructions on how to return to a previous state. In this case it would be:

• get back in the car
• head out of the parking lot
• turn right into North St
• head back down Main st
• pull into the driveway
• get out of the car
• chill out on the couch

This information is stored in the undo (or rollback) segments in the database.

A video walk through of the process is at

https://youtu.be/lmrWmH1vMPc?t=66