How can you do projection with array_agg(order by)? - postgresql

Question

I have a table with three columns: id, name and position. I want to create a JSON array as following:
[
{"id": 443, "name": "first"},
{"id": 645, "name": "second"}
]
This should be listed by the position column.
I started with the following query:
with output as
(
select id, name
from the_table
)
select array_to_json(array_agg(output))
from output
This works, great. Now I want to add the ordering. I started with this:
with output as
(
select id, name, position
from the_table
)
select array_to_json(array_agg(output order by output.position))
from output
Now the output is as following:
[
{"id": 443, "name": "first", "position": 1},
{"id": 645, "name": "second", "position": 2}
]
But I don't want the position field in the output.
I am facing a chicken-egg problem: I need the position column to be able to order on it, but I also don't want the position column, as I don't want it in the result output.
How can I fix this?
I don't think the following query is correct, as table ordering is (theoretically) not preserved between queries:
with output as
(
select id, name
from the_table
order by position
)
select array_to_json(array_agg(output))
from output

Answer 1

There are two ways (at least):
Build JSON object:
with t(x,y) as (values(1,1),(2,2))
select json_agg(json_build_object('x',t.x) order by t.y) from t;
Or delete unnecessary key:
with t(x,y) as (values(1,1),(2,2))
select json_agg((to_jsonb(t)-'y')::json order by t.y) from t;
Note that in the second case you need some type casts because - operator defined only for JSONB type.
Also note that I used direct JSON aggregation json_agg() instead of pair array_to_json(array_agg())