I had to import a large CSV file into a database, and one column must be a unique ID for a purchase. I set the type of the column to SERIAL
(yes, I know it's not actually a type) but since I already had some data in there with their own "random" purchase IDs I'm not sure about what will happen when I insert new rows.
Will the purchase ID take the values that are not already in use? Will it start after the biggest existing ID? Will it start at 1 and not care about if a value is already in use?