The slowly growing target mapping performs the following tasks:
Selects all rows
Caches the existing target as a lookup table
Compares logical key columns in the source against corresponding columns in the target lookup table
Filters out existing rows
Generates a primary key for new rows
Inserts new rows to the target
The following figure shows a mapping that the Getting Started Wizard creates when you create a slowly growing target mapping:
The slowly growing target mapping uses a Lookup and an Expression transformation to compare source data against existing target data. When you step through the Getting Started Wizard you enter the logical key columns in the source to compare against the existing target. When the Expression transformation detects source rows without matching key columns in the target, it flags the row new.
A Filter transformation passes only new rows to the Update Strategy transformation. The Update Strategy transformation marks new rows for insert and a Sequence Generator creates a new primary key value for each row written to the target.