How far are projections pushed down the execution tree?

Consider a table and a query referring to only a subset of the columns 
in that table. How early in the query evaluation is the projection 
carried out?

Are the columns to be selected filtered out as early as in the very 
access method that reads the table rows from the buffer, or are the 
projection handled later, after the whole row has been fetched by the 
access method?

Does it depend on the complexity of the query, how far down the three 
that the projection is handled out?

Thanks!