Postgres trigger function to update aggregated result in another table - postgresql

Question

I have two tables. Table x and table y. Table x gets updated everyday. I wish to update the table y as soon as new data is inserted in Table x. Table y contains the aggregated value of all the update in Table x each day.
Date is in Date type and the rest of the two column are of real type.
Table_x can be updated daily and table_y should be updated automatically.
Table x:
Date Sales product
12/12/2017 4000 2
12/12/2017 3000 1
12/12/2017 2000 1
12/12/2017 5000 3
11/12/2017 1000 3
11/12/2017 2000 4
Table y (to be as updated as shown below):
Date Sales product
12/12/2017 14000 7
11/12/2017 3000 7
I wrote the trigger function as shown below but it updates each item rather than aggregated value.
CREATE OR REPLACE FUNCTION public.rec_insert_table_y()
RETURNS trigger AS
$BODY$
BEGIN
INSERT INTO table_y ("Date","Sales","product")
SELECT NEW."Date",(sum(NEW."Sales")),(sum(NEW."product"))
GROUP BY NEW."Date";
RETURN NEW;
Trigger Function :
CREATE TRIGGER insert_into_table_y
AFTER INSERT
ON public.table_x
FOR EACH ROW
EXECUTE PROCEDURE public.rec_insert_table_y();

Answer 1

You can write a trigger that updates the aggregate value if exists or inserts if not.
Also you must be aware updates and deletes in the 'x' table:
create function y_x_trg() returns trigger
language plpgsql
as
$body$
declare
row_exists boolean;
begin
if tg_op<>'INSERT' then
update y
set sales = y.sales - old.sales,
product = y.product - old.product
where y.date = old.date
returning true into row_exists;
end if;
if tg_op<>'DELETE' then
update y
set sales = y.sales + new.sales,
product = y.product + new.product
where y.date = new.date
returning true into row_exists;
if row_exists is not true then
insert into y values (new.date, new.sales, new.product);
end if;
return new;
else
return null;
end if;
end;
$body$;
create trigger y_x_trg AFTER INSERT OR UPDATE OR DELETE ON x
FOR EACH ROW EXECUTE PROCEDURE y_x_trg();
You can see a running example at http://rextester.com/FVR79644