Why should he use CANCEL query data?

First, JOHN the user launches an application.

Second, DANIELLE user updates a row that will be included in the query.

Thirdly, the request of JOHN full.

Fourth, MALIK commits the change.

Fifth, JOHN runs its query again.


Will be First request of JOHN will use the undo data? This book says Yes (without any explanation), but I say no

Because Danielle has not committed his change, so the line interviewing JOHN has the value it had when the request has been started (see the table). Therefore, there is no need to read UNDO data (which will degrade the performance anyway).

Where I'm wrong?

Even if Eric did not commit its changes might still be written to the data file, even though it marked as "not committed". So in this case, John would require access to undo to give his point of view consistent in reading.

Tags: Database

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