Column memory usage

Import models are internally stored in a columnar compressed format. The compressed data for each column contributes to the total disk size of the file.

The primary factor of data size is a columns' cardinality. Columns with many unique values do not compress well and thus consume more space.

The example in this recipe reduced the size of the overall file, but it's the size of the individual columns being accessed by queries that, among other factors, drives query performance for import models.