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The difference between empty and non-empty EMPTY_LOB and NULL

Editor's notes:

EMPTY_LOB and NULL look alike in literal sense, but in fact, they are very different.

Not long ago, I wrote an article describing that if a table contains triggers, when importing data through IMP, the original EMPTY_LOB will be converted to NULL. A friend asked in the reply to the article, the difference between EMPTY_LOB and NULL, here is a brief description.

The execution of IMP by the LOB table containing triggers causes the EMPTY_LOB to become empty:

Http://yangtingkun.itpub.net/post/468/495024

To tell you the truth, there is actually a considerable difference between the two.

An unknown representation of one and an empty large object represented by the other. Note that empty large objects are not empty concepts:

Using IS NULL as a condition to judge, EMPTY_LOB can not be queried.

Using DBMS_LOB.GETLENGTH, you can also see the difference between the two:

Although EMPTY_LOB does not contain LOB content, the lob header information already exists, so it takes up a lot of space.

The biggest difference between the two is:

Although EMPTY_LOB does not have the content of LOB, it is ready to insert LOB content. After users get the header information of LOB, they can insert data directly.

For NULL, it is obvious that it cannot be modified directly.

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