oracle使得index变成不可用状态
oracle使得index变成不可用状态：
ALTER INDEX index_name UNUSABLE;
执行成功后，如果后期需要用到该索引的话，必须重建。重建后自动变成usable状态
另外，数据库还有2个种修改index状态叫 enable和disable：
ALTER INDEX index_name ENABLE;
ALTER INDEX index_name DISABLE;
2者之间的区别： enable和disable仅仅是针对函数索引
ENABLE applies only to a function-based index that has been disabled because a user-defined function used by the index was dropped or replaced. This clause enables such an index if these conditions are true:
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The function is currently valid
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The signature of the current function matches the signature of the function when the index was created
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The function is currently marked as DETERMINISTIC
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