PostgreSQL return result set as JSON array? - json

Question

I would like to have PostgreSQL return the result of a query as one JSON array. Given
create table t (a int primary key, b text);
insert into t values (1, 'value1');
insert into t values (2, 'value2');
insert into t values (3, 'value3');
I would like something similar to
[{"a":1,"b":"value1"},{"a":2,"b":"value2"},{"a":3,"b":"value3"}]
or
{"a":[1,2,3], "b":["value1","value2","value3"]}
(actually it would be more useful to know both). I have tried some things like
select row_to_json(row) from (select * from t) row;
select array_agg(row) from (select * from t) row;
select array_to_string(array_agg(row), '') from (select * from t) row;
And I feel I am close, but not there really. Should I be looking at other documentation except for 9.15. JSON Functions and Operators?
By the way, I am not sure about my idea. Is this a usual design decision? My thinking is that I could, of course, take the result (for example) of the first of the above 3 queries and manipulate it slightly in the application before serving it to the client, but if PostgreSQL can create the final JSON object directly, it would be simpler, because I still have not included any dependency on any JSON library in my application.

Answer 1

TL;DR
SELECT json_agg(t) FROM t
for a JSON array of objects, and
SELECT
json_build_object(
'a', json_agg(t.a),
'b', json_agg(t.b)
)
FROM t
for a JSON object of arrays.
List of objects
This section describes how to generate a JSON array of objects, with each row being converted to a single object. The result looks like this:
[{"a":1,"b":"value1"},{"a":2,"b":"value2"},{"a":3,"b":"value3"}]
9.3 and up
The json_agg function produces this result out of the box. It automatically figures out how to convert its input into JSON and aggregates it into an array.
SELECT json_agg(t) FROM t
There is no jsonb (introduced in 9.4) version of json_agg. You can either aggregate the rows into an array and then convert them:
SELECT to_jsonb(array_agg(t)) FROM t
or combine json_agg with a cast:
SELECT json_agg(t)::jsonb FROM t
My testing suggests that aggregating them into an array first is a little faster. I suspect that this is because the cast has to parse the entire JSON result.
9.2
9.2 does not have the json_agg or to_json functions, so you need to use the older array_to_json:
SELECT array_to_json(array_agg(t)) FROM t
You can optionally include a row_to_json call in the query:
SELECT array_to_json(array_agg(row_to_json(t))) FROM t
This converts each row to a JSON object, aggregates the JSON objects as an array, and then converts the array to a JSON array.
I wasn't able to discern any significant performance difference between the two.
Object of lists
This section describes how to generate a JSON object, with each key being a column in the table and each value being an array of the values of the column. It's the result that looks like this:
{"a":[1,2,3], "b":["value1","value2","value3"]}
9.5 and up
We can leverage the json_build_object function:
SELECT
json_build_object(
'a', json_agg(t.a),
'b', json_agg(t.b)
)
FROM t
You can also aggregate the columns, creating a single row, and then convert that into an object:
SELECT to_json(r)
FROM (
SELECT
json_agg(t.a) AS a,
json_agg(t.b) AS b
FROM t
) r
Note that aliasing the arrays is absolutely required to ensure that the object has the desired names.
Which one is clearer is a matter of opinion. If using the json_build_object function, I highly recommend putting one key/value pair on a line to improve readability.
You could also use array_agg in place of json_agg, but my testing indicates that json_agg is slightly faster.
There is no jsonb version of the json_build_object function. You can aggregate into a single row and convert:
SELECT to_jsonb(r)
FROM (
SELECT
array_agg(t.a) AS a,
array_agg(t.b) AS b
FROM t
) r
Unlike the other queries for this kind of result, array_agg seems to be a little faster when using to_jsonb. I suspect this is due to overhead parsing and validating the JSON result of json_agg.
Or you can use an explicit cast:
SELECT
json_build_object(
'a', json_agg(t.a),
'b', json_agg(t.b)
)::jsonb
FROM t
The to_jsonb version allows you to avoid the cast and is faster, according to my testing; again, I suspect this is due to overhead of parsing and validating the result.
9.4 and 9.3
The json_build_object function was new to 9.5, so you have to aggregate and convert to an object in previous versions:
SELECT to_json(r)
FROM (
SELECT
json_agg(t.a) AS a,
json_agg(t.b) AS b
FROM t
) r
or
SELECT to_jsonb(r)
FROM (
SELECT
array_agg(t.a) AS a,
array_agg(t.b) AS b
FROM t
) r
depending on whether you want json or jsonb.
(9.3 does not have jsonb.)
9.2
In 9.2, not even to_json exists. You must use row_to_json:
SELECT row_to_json(r)
FROM (
SELECT
array_agg(t.a) AS a,
array_agg(t.b) AS b
FROM t
) r
Documentation
Find the documentation for the JSON functions in JSON functions.
json_agg is on the aggregate functions page.
Design
If performance is important, ensure you benchmark your queries against your own schema and data, rather than trust my testing.
Whether it's a good design or not really depends on your specific application. In terms of maintainability, I don't see any particular problem. It simplifies your app code and means there's less to maintain in that portion of the app. If PG can give you exactly the result you need out of the box, the only reason I can think of to not use it would be performance considerations. Don't reinvent the wheel and all.
Nulls
Aggregate functions typically give back NULL when they operate over zero rows. If this is a possibility, you might want to use COALESCE to avoid them. A couple of examples:
SELECT COALESCE(json_agg(t), '[]'::json) FROM t
Or
SELECT to_jsonb(COALESCE(array_agg(t), ARRAY[]::t[])) FROM t
Credit to Hannes Landeholm for pointing this out

Answer 2

Also if you want selected fields from the table and aggregate them as an array:
SELECT json_agg(json_build_object('data_a',a,
'data_b',b,
)) from t;
The result will look like this:
[{'data_a':1,'data_b':'value1'}
{'data_a':2,'data_b':'value2'}]