Best way to Store and Count available stock (PostgreSQL) - sql

Question

I am not sure how to proceed with counting the stock and storing the stock in generated invoices.
Live data: http://sqlfiddle.com/#!17/20ff4/4
Table articles there the static product info is stored:
create table articles
(
prod_article_pvt_uuid uuid default uuid_generate_v1() not null
constraint proxies_pk
primary key,
prod_article_pub_uuid uuid default uuid_generate_v1mc(),
prod_article_pub_id serial not null,
t timestamp with time zone default CURRENT_TIMESTAMP(6),
archived boolean default false,
cat integer,
prod_brand_id integer,
barcode bigint,
supplier_code varchar
);
This table opens a document, and defines the type of document:
create table documents
(
sale_doc_pvt_uuid uuid default uuid_generate_v1() not null
constraint documents_pk
primary key,
sale_doc_pub_uuid uuid default uuid_generate_v1mc(),
sale_doc_alpha varchar,
t timestamp with time zone default CURRENT_TIMESTAMP(6),
archived boolean default false,
sale_type_id integer not null,
frozen boolean default false
);
create table types
(
sale_type_id serial not null
constraint types_pk
primary key,
name varchar,
draft boolean default false
);
Where each artricle is added as an item of the document:
create table items
(
sale_unit_pvt_uuid uuid default uuid_generate_v1() not null
constraint units_pk
primary key,
sale_unit_pub_uuid uuid default uuid_generate_v1mc(),
t timestamp with time zone default CURRENT_TIMESTAMP(6),
archived boolean default false,
sale_doc_pvt_uuid uuid not null,
prod_article_pvt_uuid uuid not null,
quantity numeric(16,6) not null
);
These are the available products in the warehouse:
create table products
(
whse_prod_pvt_uuid uuid default uuid_generate_v1() not null,
whse_prod_pub_uuid uuid default uuid_generate_v1mc(),
t_created timestamp with time zone default CURRENT_TIMESTAMP(6),
prod_article_pvt_uuid uuid not null,
quantity numeric(16,6) default 1,
global_ctry_id integer
);
I am trying to query this like so:
SELECT
(sum(products.quantity) - COALESCE(_items.quantity, 0)) as quantity
,articles.prod_article_pub_id as article
FROM products
LEFT JOIN articles ON products.prod_article_pvt_uuid = articles.prod_article_pvt_uuid
LEFT JOIN (
SELECT DISTINCT ON (items.prod_article_pvt_uuid)
items.prod_article_pvt_uuid
,items.sale_doc_pvt_uuid
,sum(items.quantity) as quantity
FROM items
LEFT JOIN documents ON items.sale_doc_pvt_uuid = documents.sale_doc_pvt_uuid
LEFT JOIN types ON documents.sale_type_id = types.sale_type_id
WHERE
items.archived = false
AND documents.archived = false
AND types.draft = false
GROUP BY
items.prod_article_pvt_uuid
,items.sale_doc_pvt_uuid
) AS _items ON _items.prod_article_pvt_uuid = products.prod_article_pvt_uuid
GROUP BY
articles.prod_article_pub_id
,_items.quantity
ORDER BY
article ASC,
quantity DESC
It does give me the desired result. But I am wondering if this is the correct way at going about it, cause it looks like it might struggle with millions of rows.
Basically each time I need to display the quantity available in stock I have to count every single document ever generated.
Is this a good idea?
Will it be a performance bottle neck with millions of rows?