exp,imp 小參數詳解（之三）

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IGNORE
Default: n
Specifies how object creation errors should be handled. If you accept the default, IGNORE=n, Import logs or displays object creation errors before continuing.

If you specify IGNORE=y, Import overlooks object creation errors when it attempts to create database objects, and continues without reporting the errors.

Note that only object creation errors are ignored; other errors, such as operating system, database, and SQL errors, are not ignored and may cause processing to stop.

In situations where multiple refreshes from a single export file are done with IGNORE=y, certain objects can be created multiple times (although they will have unique system-defined names). You can prevent this for certain objects (for example, constraints) by doing an import with CONSTRAINTS=n. If you do a full import with CONSTRAINTS=n, no constraints for any tables are imported.

If a table already exists and IGNORE=y, then rows are imported into existing tables without any errors or messages being given. You might want to import data into tables that already exist in order to use new storage parameters or because you have already created the table in a cluster.

If a table already exists and IGNORE=n, then errors are reported and the table is skipped with no rows inserted. Also, objects dependent on tables, such as indexes, grants, and constraints, will not be created.

Caution:

When you import into existing tables, if no column in the table is uniquely indexed, rows could be duplicated.
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當選擇 ignore=y,Import將會忽略對象的建立錯誤，並且繼續imp 操作而不報錯。
（If you specify IGNORE=y, Import overlooks object creation errors when it attempts to create database objects, and continues without reporting the errors.）

注意，僅僅對象建立錯誤會被忽略，但是其他錯誤（作業系統錯誤，資料庫錯誤，sql錯誤） 不會被忽略並可能導致進程結束。
（Note that only object creation errors are ignored; other errors, such as operating system, database, and SQL errors, are not ignored and may cause processing to stop.）

在用單個的exp檔案 對 資料庫進行多次 重新整理操作的時候，如果選擇ignore=y，某些對象可能 被建立多次，（即使他們含有唯一的系統名）。你可使用參數 constraints=y 來避免部分的重複建立。
（In situations where multiple refreshes from a single export file are done with IGNORE=y, certain objects can be created multiple times (although they will have unique system-defined names). You can prevent this for certain objects (for example, constraints) by doing an import with CONSTRAINTS=n.）
 
如果 ignore=y
當插入的表存在的時候，imp時候如果選擇 ignore=n,那麼對這個表的插入將會跳過。（依靠這個表的對象,比如index、grants、constraints都會被忽略）（If a table already exists and IGNORE=y, then rows are imported into existing tables without any errors or messages being given.）
當你對錶中的內容進行匯入的時候，如果表中沒有一列含有唯一索引，行可以被重複。
（When you import into existing tables, if no column in the table is uniquely indexed, rows could be duplicated.）

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總結:
在用單個的exp檔案 對 資料庫進行多次 重新整理操作的時候，如果選擇ignore=y，某些對象可能 被建立多次，（即使他們含有唯一的系統名）。你可使用參數 constraints=y 來避免部分的重複建立。
當插入的表存在的時候，imp時候如果選擇 ignore=n,那麼對這個表的插入將會跳過。（依靠這個表的對象,比如index、grants、constraints都會被忽略）
當你對錶中的內容進行匯入的時候，如果表中沒有一列含有唯一索引，行可以被重複。
所以對資料庫進行增量的匯入，採用imp的時候，要防止這個表沒有唯一索引，插入多次的現象。
針對exp ,imp時間點之間老庫中修改過的資料，如果要匯入新庫，也要採取別的方法來做

一般是在新庫中把這些修改的資料刪除掉，再把老庫exp 檔案匯入到新庫。（選擇 ignore=y)。