I need this pls 100 points pls make your answer long and use 2 details to support ur answer pls i really need this

Err I'm not really sure about this one.

Explanation:

Time 1: Session #1 starts a query on table A

Time 2: Session #2 updates row X in table A

Time 3: The query from session #1 arrives at row X and discovers the last updated time

(based on the SCN number) of the row to be later than Time 1 ... so the UNDO or

LOB segments are used to get the read consistent view (before image) of the row

and then the query proceeds

Time 4: Session #2 updates row Y in table A and then COMMITs (thus making it possible

for this transaction slot to be overwritten)

Time 5: Session #2 updates row Z in table B and COMMITs ... either due to space pressure

(using AUM) or bad luck (using rollback) in the read consistent view for the update

of row Y in table A at time 4 ... is overwritten (we wont examine why the overwrite

occurs at this point)

Time 6: The query from session #1 arrives at row Y and and discovers the last updated time

(based on the SCN number) of the row to be later than Time 1 ... so the UNDO or

LOB segments are examined to find the read consistent view ... BUT ... the

transaction slot containing the view ... was overwritten at time 5 ... so no read

consistent view is available ... so an ORA-1555 occurs

Explanation:

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