Partitioning on a computed column Using - Oracle 11g

I have created a table say X. of this table, a column say C3 is a computed column which will have a value based on incoming values for the other 2 columns say C1 and C2. I am using a BEFORE INSERT trigger for the same. 

The table is partitioned by Range on the computed column C3 and this is a Date column.

Now, when I try to insert data into the computed column using a Trigger, the system dumps all data into the default partition rather than creating individual partitions. Instead, when I use the computation formula directly in my Insert query; the partitioning is fine and as expected.

Any idea?

Thanx
I sounds like you are doing partitioning based on a virtual column.

virtual_column_based_partitioning