From: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
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To: | Thorsten Glaser <tg(at)evolvis(dot)org> |
Cc: | pgsql-general(at)lists(dot)postgresql(dot)org |
Subject: | Re: DISTINCT *and* ORDER BY in aggregate functions on expressions(!) |
Date: | 2023-02-28 01:48:42 |
Message-ID: | CAKFQuwYD1=6rMD+kMd=93X+wM-dJv-zScP_aDTq009DiBbKVxA@mail.gmail.com |
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Lists: | pgsql-general |
On Mon, Feb 27, 2023 at 6:22 PM Thorsten Glaser <tg(at)evolvis(dot)org> 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:
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.
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