The SELECT clause is the second phrase of an SQL query to be evaluated, following the WHERE
clause since the SELECT clause acts on the subset of rows determined by the WHERE clause. If the
data source provider supports the operations in the SELECT it is considered for delegation. The
following rules apply when determining what can be delegated to a data provider:
- All data source providers must support returning all columns from a data
- All data source providers must support returning a subset of the columns in the data
- All data source providers must support aliasing of an attribute in the SELECT
- All data source providers must be able to return an attribute from the data source
in any order.
- Aggregating functions and non-boolean operators in the SELECT clause can be delegated if the
data source provider supports them.
- Functions and non-boolean operators in a non-aggregating query cannot be delegated
to a data provider.