While reading the Redo log wiki page, I was confronted with the following statement:
For example, if a user
UPDATEs a salary-value in a table containing employee-related data, the DBMS generates a redo record containing change-vectors that describe changes to the data segment block for the table. And if the user then
COMMITs the update, Oracle generates another redo record and assigns the change a "system change number" (SCN).
In what way do change-vectors describe changes to the data? Do they contain a copy of the old and the updated data, or just the SQL statements? I also don't understand the difference between the first generated redo record and the one generated after the update is commited.