Oracle 隐藏字段是什么？
ALL_TAB_COLS differs from ALL_TAB_COLUMNS in that hidden columns are not filtered out.
What are hidden fields?
This view differs from "ALL_TAB_COLUMNS" in that hidden columns are
not filtered out.
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隐藏列是表中存在但无法选择的列。 Oracle 可能创建隐藏列的原因有多种 - 其中两个更常见的原因是：
其他各种 Oracle 功能也可能创建隐藏列，尽管这些功能往往更加深奥（即如果内存服务，interMedia 创建了几个隐藏列跟踪各种数据位）。
Hidden columns are columns that exist in the table but that cannot be selected. There are various reasons that Oracle might create a hidden column-- two of the more common reasons are
Various other Oracle features may create hidden columns as well though those features tend to be more esoteric (i.e. if memory serves, interMedia creates a couple of hidden columns to track various bits of data).
Oracle 在内部使用它们（请参阅 http://download. oracle.com/docs/cd/B10501_01/appdev.920/a96594/adobjadv.htm）并且您可以为自己的表定义此类列...在未经适当授权的情况下访问时，它们显示 NULL 而不是实际值（请参阅 http://www.techrepublic.com/article/hide-sensitive-data-with-oracle-10g-column-masking) ...
Oracle uses them internally (see http://download.oracle.com/docs/cd/B10501_01/appdev.920/a96594/adobjadv.htm) AND you can define such columns for your own tables... when accessed without proper authorization they display NULL instead of the real value (see http://www.techrepublic.com/article/hide-sensitive-data-with-oracle-10g-column-masking)...