row_to_json(), NULL values, and AS

Hi,

The following behavior does not seem self-consistent to me:

postgres=# select json_agg(row_to_json(x)) from (select *, null from
generate_series(1, 3)) x;
json_agg
-----------------------------------------------------------------------------------------------------------------------
[{"generate_series":1,"?column?":null},
{"generate_series":2,"?column?":null},
{"generate_series":3,"?column?":null}]
(1 row)

postgres=# select json_agg(row_to_json(x)) from (select *,
row_to_json(null) as jjj from generate_series(1, 3)) x;
json_agg
--------------------------------------------------------------------------------------------------------
[{"generate_series":1,"jjj":null}, {"generate_series":2,"jjj":null},
{"generate_series":3,"jjj":null}]
(1 row)

postgres=# select json_agg(row_to_json(x)) from (select *,
row_to_json(null) from generate_series(1, 3)) x;
json_agg
--------------------
[null, null, null]
(1 row)

In particular, it is unclear to me why removing the targetlist alias
in the subquery in the third example should change the result set of
the parent query.

Thanks,
Neil