When building a fact table, the final ETL step is converting the natural
keys in the new input records into the correct, contemporary surrogate
keys ETL maintains a special surrogate key lookup table for each
dimension. This table is updated whenever a new dimension entity is
created and whenever a Type 2 change occurs on an existing
dimension entity All of the required lookup tables should be pinned in
memory so that they can be randomly accessed as each incoming fact
record presents its natural keys. This is one of the reasons for making
the lookup tables separate from the original data warehouse dimension
tables.