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It occurs to me that if you have fields dependent on each other in an update statement, I’m not sure that one can guarantee the ordering (or that one needs to!).
As an example, say you had the following Update:
UPDATE Table
SET NewValue = OldValue, OldValue = NULL
Would NewValue always update first, then OldValue be nullified? Or is the state of a row (or set, or table, etc) immutable during the processing so that all the changes aren’t committed until after the changes have been calculated?
A new virtual row is created, then it replaces the existing row atomically. You have access to all the existing values until the data is committed.
Edit This is not an unusual situation, by the way.