DBAdapters and polling - Exactly how do they work?

Can someone help me clarify a point please on how a DB Adapter polls for changes on a Database please?
I'm looking at this as a starting point for an ESB process so I would be grateful for any other relevant information you could provide.
1) Is this just based on the last time the row was updated?
2) Is this going to mean a big table scan for large tables, unless you create a new index to ensure this is more efficient?
3) What happens if the Db that the adapter is looking at is down? Will entries be lost, or will they be picked up with the last updated time?
4) Do the db adapters work differently for Oracle Transparent Gateways than that when using a Generic Gateway such as MySQL?
Many thanks
Chris
(updated)

answering as best I can [ don't work for oracle ]
1) Is this just based on the last time the row was updated? - well the best is to clear the row [ ie delete it when you're done reading ] because otherwise you need a logical deletion strategy based on a sequence key -see the Database Adapter Manual for more.
2) Is this going to mean a big table scan for large tables, unless you create a new index to ensure this is more efficient? - No. Again, see the Db Adapter manual.
3) What happens if the Db that the adapter is looking at is down? Will entries be lost, or will they be picked up with the last updated time? Since [ IMHO ] the sequence key or whatever will be in the same DB as the source then it will be transactional and picked up then as well.
4) Do the db adapters work differently for Oracle Transparent Gateways than that when using a Generic Gateway such as MySQL? .... My understanding was it was all JDBC and kind of generic ....
But I agree this very important part is very badly documented indeed.
My own bugbear is the lack of accurate documentation around once-and-once only guaranteed delivery [ addressed to some extent in the current 10.1.3 SOA Best practices - but you have to hunt really hard to find it ]
The same goes for ordered message delivery.
Out of the box ESB should provide once-and-once only in-order message delivery; the documentation never tells you whether it does or not [ i.e. if a message is in the Hospital all the other messages should stop behind it ] ... do they or not ? what is the absolute conditions for this ? where is the advanced JMS/Queues discussions around this ?
Putting little Synch/Asynch diagrams doesn't really help at this kind of detail level.

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