I am loading data from a SQL database and have been selecting the relevant fields in the load statement and then using select * from the relevant table.
I wanted to see if there is any significant performance issues by selecting * rather than listing the indiviudal fields required in the load statement
Maybe. If you don't need each field then list only the fields you need. The data will be retrieved faster then if you load all the data including the fields you don't need. If you need all the fields anyway there's no performance difference.
Drawback of such approach - you will never know if some of the fields is missed from your source of data (or added as Gysbert said). Selection of specific fields instead of * gives you more control here
