The Select section of the SQL statement writes only the necessary columns

If the Select section contains columns that are not required, this forces DB2 to go to the data page to get the specific columns requested, which requires more I/O operations. In addition, if you sort this unwanted column, you need to create and pass a larger sort file, which will make sorting more expensive.

If more than one table is involved in a query, specifying extra columns also affects which joins are selected by the Optimization tool. Currently, there are 4 types of joins in Z/Os DB2 (nested loop joins, merge scan joins, Composite joins, and star joins), and there is a hash join in DB2 LUW. The optimization tool chooses various join types for different reasons. If you include extra columns that are not used at all, the optimization tool will not be able to make the best choice in the join.

The Select section of the SQL statement writes only the necessary columns