*字符大对象字符串（character large object string），clob(n)：clob 是变长字符串，最长可以达到 2,147,483,647 字节。如果只指定了 n，那么 n 的值就是最大长度。
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This approach to serialization forces a string representation of the entire object to be created in memory before a single character can be written, which is a big performance drag for large objects.
这种序列化方式强行在写入一个字符之前在内存中建立整个对象的字符串表示，对于大型对象来说，这是个非常大的性能拖累。
Another case is when you want to store a document in a database as a string or a Character Large Object (CLOB) without parsing it.
另外一种情况是，不用解析文档，而将其作为字符串或字符大对象(Character Large Object, CLOB)存储在数据库中。
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