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LOB Trigger Limitations  

You can reference object, varray, nested table, LOB, and REF columns inside triggers, but you cannot modify their values. Triggers based on actions against these types of attributes can be created.

Triggers and Mutating Tables

One recurring problem with triggers is when one tries to reference the table for which it is fired (for example, to check a date range). This self-referencing results in “ORA-04091 table ‘table_name’ is mutating, trigger/ function may not see it.” Usually, this situation can be remedied in one of three ways: by using a temporary PL/SQL table to hold values, by using a temporary table to hold values, or by using a view. The key here is to remove the values to be selected against from the table before you attempt the operation, and then refer to this remote source for any value checking. It might also be possible to create an index that any selects will reference. Any solution that moves the data from the table to a secondary source that can then be used in place of the table itself should correct this problem.

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