Backfilling data and partitions - looking for best practice/advice

Assume that the table is hierarchically partitioned by day for the last 3 months, month for the past year, and year from then onwards, and the table is sales.f_sales. Assume that the new records are really new, so no updates required. And all new records refer to the year 2021. And the new records sit in a landing-zone table landing.f_sales. And the driving column for the element of time is sales_dt.

Step 1: create a staging table like the base table, and partitioned the same way:

CREATE TABLE sales.f_sales_stg LIKE sales.f_sales INCLUDING PROJECTIONS;

Step 2 : Create a year partition for the year affected by the change in one transaction. That should create one ROS container, with all 365 partition keys of 2021 combined.

INSERT INTO sales.f_sales_stg
SELECT * FROM landing.f_sales
UNION ALL
SELECT * FROM sales.f_sales WHERE sales_dt BETWEEN DATE '2021-01-01' AND DATE '2021-12-31'

Step 3: Swap the right partitions between the staging table and the base table:

SELECT 
  SWAP_PARTITIONS_BETWEEN_TABLES(
    'sales.f_sales_stg'
  , '2021-01-01'
  , '2021-12-31'
  , 'sales.f_sales’
);

Thanks marcothesane. I will give that a try. It does imply there's only 1 writer process, to avoid data getting lost between steps 2 and 3.