Reference partition with Trigger

Question

This post is an extended version if my previous post -Dynamic Reference partition by Trigger on Feb 24
I need to create a Dynamic Reference partition with Trigger.
Requirement:-
1. It should be a list partition
2. It should be a referencing Partition (There are 2 tables with Parent Child relation)
3. It should be dynamic(trigger) (Whenever new value enter to the List, It should be create a New partition by trigger, As per my understanding interval partitioning is dynamic but it will work only for range partition. )
In other way, I can manually create both the tables with List partition initially itself, but when the new value is added to the List the table supposed to add the new partition. List value is unknown at the time of creation. Since the List value may increase and in future it will be more than five, if it is five we need five partition so partition with a default clause also not possible here.
Almost I have completed this task, but facing some issues. Please find the example i have done. (Please note that it is just an example and not my actual requirement)
==============================================================================================================
--TABLE 1
create table customers
cust_id number primary key,
cust_name varchar2(200),
rating varchar2(1) not null
partition by list (rating)
partition pA values ('A'),
partition pB values ('B')
--TABLE 2
create table sales
sales_id number primary key,
cust_id number not null,
sales_amt number,
constraint fk_sales_01
foreign key (cust_id)
references customers
partition by reference (fk_sales_01);
--TRIGGER
CREATE OR REPLACE TRIGGER CUST_INSERT
BEFORE INSERT ON CUSTOMERS FOR EACH ROW
DECLARE
V_PARTITION_COUNT NUMBER;
V_PART_NAME VARCHAR2 (100) := :NEW.RATING;
PRAGMA AUTONOMOUS_TRANSACTION;
BEGIN
SELECT COUNT (PARTITION_NAME)
INTO V_PARTITION_COUNT
FROM USER_TAB_PARTITIONS
WHERE TABLE_NAME = 'CUSTOMERS' AND PARTITION_NAME = V_PART_NAME;
IF V_PARTITION_COUNT = 0 THEN
EXECUTE IMMEDIATE 'ALTER TABLE CUSTOMERS ADD PARTITION '||V_PART_NAME||' VALUES ('''||V_PART_NAME||''')' ;
END IF;
END;
--INSERT TO CUSTOMER
insert into customers values (111, 'OOO', 'C');-- A and B are already in the Customer table List along with the create script, so I am inserting new row with 'C'
==============================================================================================================
When i am inserting the new value 'C' to the customer table it supposed to create a new partition along with new row to the table.
But I am getting an error like "ORA-14400: inserted partition key does not map to any partition", like partition is not exist.
But If I execute the same insert statement again it is inserting the row.
That means Even though it is a before insert trigger, insert is trying to happen before the partition creation.
Since it is before insert trigger My expectation is
a) Create a Partition first
b) Insert the record second
But actual behavior is
a) Try to insert first and failing
b) Creating the partition second
That is the reason when I am trying second time the record is getting inserted.
Can anyone help me on this, Thanks in advance.
Shijo

Answer 1

You can't do this with a trigger. And you really, really don't want to.
As you've discovered, Oracle performs a number of checks to determine whether an insert is valid before any triggers are executed. It is going to discover that there is no appropriate partition before your trigger runs. Even if you somehow worked around that problem, it's likely that you'd end up in a rather problematic position from a lock standpoint since you'd be trying to do DDL on the object in the autonomous transaction while doing an insert likely from a stored procedure that would itself be made invalid because of the DDL in another-- that's quite likely to cause an error anyway. And that's before we get into the effects of doing DDL on other sessions running at the same time.
Why do you believe that you need to add partitions dynamically? If you are doing ETL into a data warehouse, it makes sense to add any partitions you need at the beginning of the load before you start inserting the data, not in a trigger. If this is some sort of OLTP application, it doesn't make sense to do DDL while the application is running. I could see potentially having a default partition and then having a nightly job that does a split partition to add whatever list partitions you need but that would strike me as a corner case at best. If you really don't know what the possible values are and users are constantly creating new ones, list partitioning seems like a poor choice to me-- wouldn't a hash partition make more sense?
Justin