不容易理解的2个字段，Jonathan Lewis的解释
http://www.dbforums.com/archive/index.php/t-1314288.html
[@more@]Jonathan Lewis
The slots in the transaction table in the undo segment header
are continuously being reused. Each time a slot it re-used,
its sequence number (xidsqn) goes up.
Similarly, undo blocks are re-used (though it takes more
time to get round to re-using them than transaction slots).
Each time an undo block is "newed" and reused, its
sequence number UBASQN goes up.
are continuously being reused. Each time a slot it re-used,
its sequence number (xidsqn) goes up.
Similarly, undo blocks are re-used (though it takes more
time to get round to re-using them than transaction slots).
Each time an undo block is "newed" and reused, its
sequence number UBASQN goes up.
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