什么是BLOCK CLEANOUT！[@more@]BLOCK CLEANOUT When a data block is altered, important transaction information is written to the block along with the data changes. When the transaction is committed, the block is not rewritten to reflect the new transaction state. The next transaction, whether a SELECT or INSERT/UPDATE/DELETE, will ‘discover’ the out of date information and update the block to reflect the committed state of the previous transaction. This is called ‘delayed block cleanout.’ It is possible that the block will never be touched again and contain transaction information that is days, weeks, months or even years old.
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