select * or individual columns

Question

Is there any difference in performance between

select *
from table name

and

select [col1]
      ,[col2]
       ......
      ,[coln]
from table name

Answer 1

It is a SQL antipattern to use SELECT *. It is faster for the database when you specify columns (it doesn't have to look them up) and more importantly, you should not ever specify more columns than you actually need. If you have a join in your query you have at least one column you don't need (the join column) and thus SELECT * is always slower to return records in a query with a join since it is returning more infomation than necessary.

Now all this sounds like a small improvement and in a small system it might be, but as the database grows and gets busy, the performance implications become bigger. There is no excuse for using SELECT *.

SELECT * is also bad for maintenance especially if you use it to insert records - always specify both the columns you are inserting to and the fields from the select in an INSERT statement that uses a SELECT. It will break if you change the table structure. You may also end up showing the user columns you don't want them to see such as the GUID added for replication.

If you use SELECT * in a view (at least in SQL Server) the view will still not automatically update in the case of a change to the underlying tables. If someone gets the silly idea to re-arrange the column order in the table (yeah I know you shouldn't but people do this on occasion) using SELECT * might mean that data will show up in the wrong columns in reports or inserts which can cause problems where things might be misinterpreted. I can think of one case where two columns were swapped in a staging table and the social security number became the amount we intended to pay the person giving a speech. You can see how that might really muck up the accounting except we didn't use SELECT * so we were safe because the columns retained the same name.

I want to note that you don't even save much development time by using SELECT *. It takes me a max of about 15 seconds more to use the table names even in a large table as I drag them over from the object browser in SQL Server (you can get all the columns in one step).