How to aggregate rows of jsonb elements in postgres?

I want to know if there is any efficient way to aggregate several rows of a JSONB column, consider this table structure:

PERFORMANCE(user_id INTEGER, stat_date DATE, metrics JSONB)

With rows like this:

1, 2017-01-01, {"speed":10, "distance":120, "time":5}
1, 2017-01-02, {"speed":15, "distance":150, "time":8}
1, 2017-01-03, {"speed":9, "distance":90}
2, 2017-01-01, {"speed":15, "distance":150, "time":8}

I would like to aggregate by SUM each key inside "metrics" column by user_id, so the output looks like this:

1, {"speed":34, "distance":360, "time":13}
2, {"speed":15, "distance":150, "time":8}

One option is to use jsonb_each but that will explode the column and I was wondering if there is a better option.

EDIT: It's important to note that I don't know every key inside the JSONB column, so I can't explicitly aggregate them.

Thanks

Solution

One way is just get values from jsonb data, sum they using group by user_id and then make jsonb data again:

select user_id, jsonb_build_object('speed', sum((metrics->>'speed')::numeric), 'distance', sum((metrics->>'distance')::numeric) , 'time', sum((metrics->>'time')::numeric) )
from t 
group by user_id

Edit

Well, in this case I think as you said, you can use jsonb_each function (May be there is better ways too, but I can't see it now). I use jsonb_each_text here, I don't know what you mean but that will explode the column, you can use jsonb_object_agg for "building" jsonb object again, something like this:

with your_table(user_id  , stat_date  , metrics) as(
    select 1, '2017-01-01'::date, '{"speed":10, "distance":120, "time":5}'::jsonb union all
    select 1, '2017-01-02'::date, '{"speed":15, "distance":150, "time":8}'::jsonb union all
    select 1, '2017-01-02'::date, '{"bla":8}'::jsonb union all
    select 4, '2017-01-02'::date, '{"bla":8}'::jsonb union all
    select 1, '2017-01-03'::date, '{"speed":9, "distance":90}'::jsonb union all
    select 2, '2017-01-01'::date, '{"speed":15, "distance":150, "time":8}'::jsonb  
)

select user_id, jsonb_object_agg(k, val) from (
    select user_id, k, sum(v::numeric) as val from your_table join lateral  jsonb_each_text(metrics) j(k,v) on true
    group by user_id, k
) tt
group by user_id