Redshift copy command append, replace or upsert?

Question

Suppose I run the Redshift COPY command for a table where existing data. Then does the command:

1. Appends the data to the existing table?
2. Wipes clean existing data and add the new data?
3. Upserts the data. i.e., UPDATE if data with the same primary key is present in table or INSERT otherwise

Answer 1

The COPY command always appends data to a table.

Answer 2

In order to effectively upsert in Redshift using "copy" command, you need first to load your data (from your copy) to a staging table then run some sql on redshift to process this data.

AWS have documented an approach here https://docs.aws.amazon.com/redshift/latest/dg/t_updating-inserting-using-staging-tables-.html