Limit query by count distinct column values - postgresql

Question

I have a table with people, something like this:
ID PersonId SomeAttribute
1 1 yellow
2 1 red
3 2 yellow
4 3 green
5 3 black
6 3 purple
7 4 white
Previously I was returning all of Persons to API as seperate objects. So if user set limit to 3, I was just setting query maxResults in hibernate to 3 and returning:
{"PersonID": 1, "attr":"yellow"}
{"PersonID": 1, "attr":"red"}
{"PersonID": 2, "attr":"yellow"}
and if someone specify limit to 3 and page 2(setMaxResult(3), setFirstResult(6) it would be:
{"PersonID": 3, "attr":"green"}
{"PersonID": 3, "attr":"black"}
{"PersonID": 3, "attr":"purple"}
But now I want to select people and combine then into one json object to look like this:
{
"PersonID":3,
"attrs": [
{"attr":"green"},
{"attr":"black"},
{"attr":"purple"}
]
}
And here is the problem. Is there any possibility in postgresql or hibernate to set limit not by number of rows but to number of distinct people ids, because if user specifies limit to 4 I should return person1, 2, 3 and 4, but in my current limiting mechanism I will return person1 with 2 attributes, person2 and person3 with only one attribute. Same problem with pagination, now I can return half of a person3 array attrs on one page and another half on next page.

Answer 1

You can use row_number to simulate LIMIT:
-- Test data
CREATE TABLE person AS
WITH tmp ("ID", "PersonId", "SomeAttribute") AS (
VALUES
(1, 1, 'yellow'::TEXT),
(2, 1, 'red'),
(3, 2, 'yellow'),
(4, 3, 'green'),
(5, 3, 'black'),
(6, 3, 'purple'),
(7, 4, 'white')
)
SELECT * FROM tmp;
-- Returning as a normal column (limit by someAttribute size)
SELECT * FROM (
select
"PersonId",
"SomeAttribute",
row_number() OVER(PARTITION BY "PersonId" ORDER BY "PersonId") AS rownum
from
person) as tmp
WHERE rownum <= 3;
-- Returning as a normal column (overall limit)
SELECT * FROM (
select
"PersonId",
"SomeAttribute",
row_number() OVER(ORDER BY "PersonId") AS rownum
from
person) as tmp
WHERE rownum <= 4;
-- Returning as a JSON column (limit by someAttribute size)
SELECT "PersonId", json_object_agg('color', "SomeAttribute") AS attributes FROM (
select
"PersonId",
"SomeAttribute",
row_number() OVER(PARTITION BY "PersonId" ORDER BY "PersonId") AS rownum
from
person) as tmp
WHERE rownum <= 3 GROUP BY "PersonId";
-- Returning as a JSON column (limit by person)
SELECT "PersonId", json_object_agg('color', "SomeAttribute") AS attributes FROM (
select
"PersonId",
"SomeAttribute"
from
person) as tmp
GROUP BY "PersonId"
LIMIT 4;
In this case, of course, you must use a native query, but this is a small trade-off IMHO.
More info here and here.

Answer 2

I'm assuming you have another Person table. With JPA, you should do the query on Person table(one side), not on the PersonColor(many side).Then the limit will be applied on number of rows of Person then
If you don't have the Person table and can't modify the DB, what you can do is use SQL and Group By PersonId, and concatenate colors
select PersonId, array_agg(Color) FROM my_table group by PersonId limit 2
SQL Fiddle

Answer 3

Thank you guys. After I realize that it could not be done with one query I just do sth like
temp_query = select distinct x.person_id from (my_original_query) x
with user specific page/per_page
and then:
my_original_query += " AND person_id in (temp_query_results)