Hi Walter,
On slide 46 of http://www.slideshare.net/ibsurgeon/3-how-transactionswork
we consider how transaction 20 view record versions.
It's important to note that transaction 20 is a snapshot.
>Tx16 did the INSERT?
Yes, it created original version of Record 1.
>Tx12 can do a COMMIT although it had started before Tx16 and Tx18 is
still active?
Sure, why not?
>Tx25 can change what Tx14 did although Tx14 is still active?
Yes, it looks a bit confusing, but this is a snapshot view which
highlights that Tx 20 CANNOT see record versions from a) active
transactions, b) transactions with Number > 20.
It means that for snapshot transaction 20 all transactions above its
number look like active, and as a result, it cannot view record versions
from both from Tx 14, which was active at the moment when snapshot copy
of TIP for Tx25 was taken, and it also cannot view record version from
Tx 25, which, obviously was created after Tx 14 was committed. But,
again, for Tx 20 these both record versions are invisible. Tx20, as a
snapshot, thinks that Tx is active, but in reality it was committed.
Regards,
Alexey Kovyazin
IBSurgeon
Hi Alexey
Yes, that's right, I can not understand well the page 46. I suppose
that R1 means "record version 1", R2 means "record version 2", etc.
Tx16 did the INSERT?
Tx12 can do a COMMIT although it had started before Tx16 and Tx18 is
still active?
Tx25 can change what Tx14 did although Tx14 is still active?
These things had confused me.
Of course, I think that I am not understanding rightly the graph,
that's why some words can help to do it clear.
