undo系列学习之深入浅出事务槽
Oracle数据块头部有个事务槽（ITL）。当多个事务槽同时修改数据块，而且，此时，pctfree（数据块空闲空间的比例）不足10%，则会出
Oracle数据块头部有个事务槽（ITL）。当多个事务槽同时修改数据块，而且，此时，pctfree（数据块空闲空间的比例）不足10%，则会出现ITL争用。这种现象容易发生在update和delete身上。因为，insert时，oracle会优先分散地插入其他空闲块。如：
看一下表a有多少个事务槽：
缺省下是1个，，最多可以有255个
看一下表a用了多少个数据块：
可知：表a在4号文件上，第460个数据块。
可以把id=2，数据块为460的4号文件dump出来，看一下块头的ITL：
ITL部分内容摘入如下：
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