Aggregate hstore in Postgres within GROUP BY - sql

Question

I have data with an hstore like this:
|brand|account|likes|views |
|-----|-------|-----|----------------------|
|Ford |ford_uk|1 |"3"=>"100" |
|Ford |ford_us|2 |"3"=>"200", "5"=>"10" |
|Jeep |jeep_uk|3 |"3"=>"300" |
|Jeep |jeep_us|4 |"3"=>"400", "5"=>"20" |
I would like to be able to sum the hstores by key, grouped by brand:
|brand|likes|views |
|-----|-----|----------------------|
|Ford |3 |"3"=>"300", "5"=>"10" |
|Jeep |7 |"3"=>"700", "5"=>"20" |
This answer gives a good solution for how to do this without a GROUP BY. Adapting it to this situation gives something like:
SELECT
sum(likes) AS total_likes,
(SELECT hstore(array_agg(key), array_agg(value::text))
FROM (
SELECT s.key, sum(s.value::integer)
FROM (
SELECT((each(views)).*)
) AS s(key, value)
GROUP BY key
) x(key, value)) AS total_views
FROM my_table
GROUP BY brand
However this gives:
ERROR: subquery uses ungrouped column "my_table.views" from outer query
Any help appreciated!

Answer 1

It is because of using views column without aggregate function in the group by query.
Very quick workaround:
with my_table(brand,account,likes,views) as (
values
('Ford', 'ford_uk', 1, '"3"=>"100"'::hstore),
('Ford', 'ford_uk', 2, '"3"=>"200", "5"=>"10"'),
('Jeep', 'jeep_uk', 3, '"3"=>"300"'::hstore),
('Jeep', 'jeep_uk', 4, '"3"=>"400", "5"=>"20"'))
SELECT
brand,
sum(likes) AS total_likes,
(SELECT hstore(array_agg(key), array_agg(value::text))
FROM (
SELECT s.key, sum(s.value::integer)
FROM
unnest(array_agg(views)) AS h, --<< aggregate views according to the group by, then unnest it into the table
each(h) as s(key,value)
GROUP BY key
) x(key, value)) AS total_views
FROM my_table
GROUP BY brand
Update
Also you can to create the aggregate for such tasks:
--drop aggregate if exists hstore_sum(hstore);
--drop function if exists hstore_sum_ffunc(hstore[]);
create function hstore_sum_ffunc(hstore[]) returns hstore language sql immutable as $$
select hstore(array_agg(key), array_agg(value::text))
from
(select s.key, sum(s.value::numeric) as value
from unnest($1) as h, each(h) as s(key, value) group by s.key) as t
$$;
create aggregate hstore_sum(hstore)
(
SFUNC = array_append,
STYPE = hstore[],
FINALFUNC = hstore_sum_ffunc,
INITCOND = '{}'
);
After that your query will be simpler and more "canonical":
select
brand,
sum(likes) as total_likes,
hstore_sum(views) as total_views
from my_table
group by brand;
Update 2
Even without create aggregate the function hstore_sum_ffunc could be useful:
select
brand,
sum(likes) as total_likes,
hstore_sum_ffunc(array_agg(views)) as total_views
from my_table
group by brand;

Answer 2

If you create an aggregate for hstore, this gets a bit easier:
create aggregate hstore_agg(hstore)
(
sfunc = hs_concat(hstore, hstore),
stype = hstore
);
Then you can do this:
with totals as (
select t1.brand,
hstore(k, sum(v::int)::text) as views
from my_table t1, each(views) x(k,v)
group by brand, k
)
select brand,
(select sum(likes) from my_table t2 where t1.brand = t2.brand) as likes,
hstore_agg(views) as views
from totals t1
group by brand;
Another option is to move the co-related sub-query which might be slow into a CTE:
with vals as (
select t1.brand,
hstore(k, sum(v::int)::text) as views
from my_table t1, each(views) x(k,v)
group by brand, k
), view_totals as (
select brand,
hstore_agg(views) as views
from vals
group by brand
), like_totals as (
select brand,
sum(likes) as likes
from my_table
group by brand
)
select vt.brand,
lt.likes,
vt.views
from view_totals vt
join like_totals lt on vt.brand = lt.brand
order by brand;