What happens if I add new columns to my snapshot query?

When the columns of your source query changes, dbt will attempt to reconcile this change in the destination snapshot table. dbt does this by:

1. Creating new columns from the source query in the destination table
2. Expanding the size of string types where necessary (eg. varchars on Redshift)

dbt will not delete columns in the destination snapshot table if they are removed from the source query. It will also not change the type of a column beyond expanding the size of varchar columns. That is, if a string column is changed to a date column in the snapshot source query, dbt will not attempt to change the type of the column in the destination table.