extra TARGETENTRY when group by function( col )?

Given:

            create table t1 ( c1 timestamp without time zone, c2 int );

            explain verbose select to_char ( c1, ‘<whatever>’ ) from t1 group by to_char ( c1, ‘<whatever>’ );

does any one know why the SEQSCAN node has TWO TARGETENTRYs?

Why do we project “c1” as well as “to_char( c1 … )” ?

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For cases where we compute an aggregate, such as:

            select min(c1), to_char(c1,…) from t1 group by to_char( c1,…)

it would make (some kind of) sense to project out “c1”.

But can’t the planner/optimizer distinguish these cases on the basis that ‘min’ is an aggregate function and ‘to_char’ is not?

The planner seems to be computing a minimal target list of columns (no expressions) first.  Guess I’m asking why it isn’t replaced.