Aggregate Count Over Variable Number of JSON Keys in PostgreSQL

aggregatejsonpostgresqlpostgresql-9.4

How can one get a list of json objects like this:

[
    {"band": "beatles", "bass": 1, "drums": 1, "guitar": 2},
    {"band": "metallica", "bass": 1, "drums": 1, "guitar": 2, "vocals": 1}
]

from this data on Postgresql 9.4?

drop table if exists people;
create temp table people(pk serial primary key, band character varying(15), fname text, tag character varying(15));

insert into people(band, fname, tag) 
values
('beatles', 'john', 'guitar'), 
('beatles', 'paul', 'bass'),
('beatles', 'ringo', 'drums'),
('beatles', 'george', 'guitar'),
('metallica', 'james', 'vocals'),
('metallica', 'lars', 'drum'),
('metallica', 'kirk', 'guitar'),
('metallica', 'robert', 'bass'),
('metallica', 'marty', 'guitar');

--Summary table
select band, tag, count(*)  as ct from people  group by band, tag order by band, tag

This is my closest attempt:

SELECT band, tags from (
    select band, row_to_json(row(tag, count(*))) as tags from people group by band, tag
) t

-- Which leads me towards:
SELECT band, array_agg(tags) from (
    select band, row_to_json(row(tag, count(*))) as tags from people group by band, tag
) t
GROUP BY t.band;

--This is close, but from *{f1:v1, f2:v2}* I need *{v1:v2}*

Here is how I arrived to that one:

select array(SELECT row_to_json(r) from (
    SELECT band, array_agg(tag) from people group by band
    ) r
)

select row_to_json(r) from (
    SELECT band, array_agg(tag), count(*) from people group by band
    ) r;

select row_to_json(r) from (
    SELECT band, json_agg(tag), sum(ct) from (
        select band, tag, count(*)  as ct from people  group by band, tag
    ) x group by x.band
) r;

Best Answer

The only one thing which I not found is the function/operator for concatenation of JSON objects, so lets create it:

create function json_concat(in json, in json) returns json as $$
  select
    (
      left($1::text, length($1::text)-1) || ', ' ||
      right($2::text, length($2::text)-1)
    )::json; 
$$ immutable language sql;

After this the final query would be:

with t as (
  select band, tag, count(*) as ct 
  from people
  group by band, tag
  order by band, tag)

select json_agg(x)
from (
  select json_concat(json_build_object('band', band), json_object_agg(tag, ct)) as x
  from t 
  group by band) tt;

and result is

[{"band" : "metallica",  "bass" : 1, "drums" : 1, "guitar" : 2, "vocals" : 1 }, {"band" : "beatles",  "bass" : 1, "drums" : 1, "guitar" : 2 }]

The query you have

You could simplify your query using a WINDOW clause, but that's just shortening the syntax, not changing the query plan.

SELECT id, trans_ref_no, amount, trans_date, entity_id
     , SUM(amount) OVER w AS trans_total
     , COUNT(*)    OVER w AS trans_count
FROM   transactiondb
WINDOW w AS (PARTITION BY entity_id, date_trunc('month',trans_date)
             ORDER BY trans_date
             ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING);

Also using the slightly faster count(*), since id is certainly defined NOT NULL?
And you don't need to ORDER BY entity_id since you already PARTITION BY entity_id

You can simplify further, though:
Don't add ORDER BY to the window definition at all, it's not relevant to your query. Then you don't need to define a custom window frame, either:

SELECT id, trans_ref_no, amount, trans_date, entity_id
     , SUM(amount) OVER w AS trans_total
     , COUNT(*)    OVER w AS trans_count
FROM   transactiondb
WINDOW w AS (PARTITION BY entity_id, date_trunc('month',trans_date);

Simpler, faster, but still just a better version of what you have, with static months.

The query you might want

... is not clearly defined, so I'll build on these assumptions:

Count transactions and amount for every 30-day period within the first and last transaction of any entity_id. Exclude leading and trailing periods without activity, but include all possible 30-day periods within those outer bounds.

SELECT entity_id, trans_date
     , COALESCE(sum(daily_amount) OVER w, 0) AS trans_total
     , COALESCE(sum(daily_count)  OVER w, 0) AS trans_count
FROM  (
   SELECT entity_id
        , generate_series (min(trans_date)::timestamp
                         , GREATEST(min(trans_date), max(trans_date) - 29)::timestamp
                         , interval '1 day')::date AS trans_date
   FROM   transactiondb 
   GROUP  BY 1
   ) x
LEFT JOIN (
   SELECT entity_id, trans_date
        , sum(amount) AS daily_amount, count(*) AS daily_count
   FROM   transactiondb
   GROUP  BY 1, 2
   ) t USING (entity_id, trans_date)
WINDOW w AS (PARTITION BY entity_id ORDER BY trans_date
             ROWS BETWEEN CURRENT ROW AND 29 FOLLOWING);

This lists all 30-day periods for each entity_id with your aggregates and with trans_date being the first day (incl.) of the period. To get values for each individual row join to the base table once more ...

The basic difficulty is the same as discussed here:

Referencing current row in FILTER clause of window function

The frame definition of a window cannot depend on values of the current row.

And rather call generate_series() with timestamp input:

Generating time series between two dates in PostgreSQL

The query you actually want

After question update and discussion:
Accumulate rows of the same entity_id in a 30-day window starting at each actual transaction.

Since your data is distributed sparsely, it should be more efficient to run a self-join with a range condition, all the more since Postgres 9.1 does not have LATERAL joins, yet:

SELECT t0.id, t0.amount, t0.trans_date, t0.entity_id
     , sum(t1.amount) AS trans_total, count(*) AS trans_count
FROM   transactiondb t0
JOIN   transactiondb t1 USING (entity_id)
WHERE  t1.trans_date >= t0.trans_date
AND    t1.trans_date <  t0.trans_date + 30  -- exclude upper bound
-- AND    t0.entity_id = 114284  -- or pick a single entity ...
GROUP  BY t0.id  -- is PK!
ORDER  BY t0.trans_date, t0.id

SQL Fiddle.

A rolling window could only make sense (with respect to performance) with data for most days.

This does not aggregate duplicates on (trans_date, entity_id) per day, but all rows of the same day are always included in the 30-day window.

For a big table, a covering index like this could help quite a bit:

CREATE INDEX transactiondb_foo_idx
ON transactiondb (entity_id, trans_date, amount);

The last column amount is only useful if you get index-only scans out of it. Else drop it.

But it's not going to be used while you select the whole table anyway. It would support queries for a small subset.

This is my closest attempt:

Here is how I arrived to that one:

Best Answer

Related Solutions

PostgreSQL – Aggregate Objects into JSON Array

PostgreSQL – How to Calculate Rolling Sum, Count, and Average Over Date Interval

The query you have

The query you might want

The query you actually want

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