Re: DISTINCT *and* ORDER BY in aggregate functions on expressions(!)

["David G. Johnston" <david.g.johnston@xxxxxxxxx>]

On Mon, Feb 27, 2023 at 6:22 PM Thorsten Glaser <tg@xxxxxxxxxxx> wrote:

Not about the DISTINCTs. I haven’t used JOIN much (usually WHERE
as inner join) nor on tables this massive, and this is my second
foray into aggregate functions only.

Fair.

Consider this then as a jumping point to a more precise query form:

https://dbfiddle.uk/wz2MKtuF

create table base (base_id integer primary key, base_val text not null);

insert into base values (1, 'one');

create table subone (subone_id serial primary key, base_id integer, subone_value text not null);

insert into subone (base_id, subone_value) values (1, 'subone-one'), (1, 'subone-two');

create table subtwo (subtwo_id serial primary key, base_id integer, subtwo_value text not null);

insert into subtwo (base_id, subtwo_value) values (1, 'subtwo-one'), (1, 'subtwo-two');

--BAD cross joining going on with multiple one-to-many relationships

select * from base join subone using (base_id) join subtwo using (base_id); -- not good

--GOOD, only joining one-to-one relationships

select jsonb_build_object('base_id', base_id, 'subone_arr', subone_arr, 'subtwo_arr', subtwo_arr)
from base
join lateral (select json_agg(jsonb_build_object('key', subone_value) order by subone_value)
               from subone where subone.base_id = base.base_id) as so (subone_arr) on true
join (select base_id, json_agg(subtwo_value order by subtwo_value desc)
               from subtwo group by base_id) as st (base_id, subtwo_arr) using (base_id)

I used a mix of forms in the two joins, and there are other variants, but the basic concept holds - produce single rows in subqueries then join those various single rows together to produce your desired json output.

David J.