When you design
your data warehouse, you need to decide what type of information to store in
targets. As part of your target table design, you need to determine whether to
maintain all the historic data or just the most recent changes.
For example, you
might have a target table, T_CUSTOMERS that contains customer data. When a
customer address changes, you may want to save the original address in the
table, instead of updating that portion of the customer record. In this case,
you would create a new record containing the updated address, and preserve the
original record with the old customer address. This illustrates how you might
store historical information in a target table. However, if you want the
T_CUSTOMERS table to be a snapshot of current customer data, you would update
the existing customer record and lose the original address.
The model you
choose constitutes your update strategy, how to handle changes to
existing records. In PowerMart and PowerCenter, you set your update strategy at
two different levels:
a) Within a session:
When you configure a session, you can
instruct the Informatica Server to either treat all records in the same way
(for example, treat all records as inserts), or use instructions coded into the
session mapping to flag records for different database operations.
b) Within a mapping: Within
a mapping, you use the Update Strategy transformation to flag records for
insert, delete, update, or reject.
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