[pbalau]

> I have row X that takes up 1kb. I replace it with row Y that takes up 500b. I then write row Z after row Y that takes up 500b. If you want to rollback the original transaction, row X will no longer fit in its original spot because row Z is now taking up part of the space it used to occupy.

But this only can happen if replace X with Y and add Z are in the same transaction. If you rollback the transaction, then you start by removing Z, then replacing Y with X. What I'm missing here?

[malisper]

> But this only can happen if replace X with Y and add Z are in the same transaction. If you rollback the transaction, then you start by removing Z, then replacing Y with X. What I'm missing here?

Z can be in a different transaction than X and Y. If two transactions run concurrently, one that replaces X with Y and a second one that inserts Z, the above scenario can happen.

[pbalau]

No it cant. If they run concurrently, Z has no notion of Y happening. This is the whole point of transactions.

[shock-value]

Transactions also have no "notion" of storage implementation details, so your point really isn't relevant to the topic at hand. They operate at a higher level of abstraction.