Thank you very much for your answer Alexey, I was very bussy these days. Looking at the page 46 it seems more understandable now.
Greetings. Walter. On Wed, Mar 25, 2015 at 2:02 PM, Alexey Kovyazin a...@ib-aid.com [firebird-support] <firebird-support@yahoogroups.com> wrote: > > > 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. > > > > >
