PostgreSQL joining using JSONB

Question

I have this SQL:

CREATE TABLE test(id SERIAL PRIMARY KEY, data JSONB);

INSERT INTO test(data) VALUES
   ('{"parent":null,"children":[2,3]}'),
   ('{"parent":1,   "children":[4,5]}'),
   ('{"parent":1,   "children":[]}'),
   ('{"parent":2,   "children":[]}'),
   ('{"parent":2,   "children":[]}');

That would give:

 id |                 data                 
----+--------------------------------------
  1 | {"parent": null, "children": [2, 3]}
  2 | {"parent": 1, "children": [4, 5]}
  3 | {"parent": 1, "children": []}
  4 | {"parent": 2, "children": []}
  5 | {"parent": 2, "children": []}

When doing normal one to many, it would show something like this:

SELECT * 
FROM test x1
  LEFT JOIN test x2
    ON x1.id = (x2.data->>'parent')::INT;
 id |                 data                 | id |               data                
----+--------------------------------------+----+-----------------------------------
  1 | {"parent": null, "children": [2, 3]} |  2 | {"parent": 1, "children": [4, 5]}
  1 | {"parent": null, "children": [2, 3]} |  3 | {"parent": 1, "children": []}
  2 | {"parent": 1, "children": [4, 5]}    |  4 | {"parent": 2, "children": []}
  2 | {"parent": 1, "children": [4, 5]}    |  5 | {"parent": 2, "children": []}
  5 | {"parent": 2, "children": []}        |    | 
  4 | {"parent": 2, "children": []}        |    | 
  3 | {"parent": 1, "children": []}        |    |

How to join based on children (using LEFT JOIN or WHERE IN)? I've tried:

SELECT data->>'children' FROM test;
 ?column? 
----------
 [2, 3]
 [4, 5]
 []
 []
 []

SELECT json_array_elements((data->>'children')::TEXT) FROM t...
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

SELECT json_array_elements((data->>'children')::JSONB) FROM ...
               ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

SELECT json_to_record((data->>'children')::JSON) FROM test;
ERROR:  function returning record called in context that cannot accept type record
HINT:  Try calling the function in the FROM clause using a column definition list.

SELECT * FROM json_to_record((test.data->>'children')::JSON);
ERROR:  missing FROM-clause entry for table "test"
LINE 1: SELECT * FROM json_to_record((test.data->>'children')::JSON)...

Erwin Brandstetter · Accepted Answer · 2021-03-30 13:39:55Z

This would be more efficient:

With `jsonb` and `jsonb_array_elements_text()` in pg 9.4+

EXPLAIN 
SELECT p.id AS p_id, p.data
     , c.id AS c_id, c.data
FROM   test p
LEFT   JOIN LATERAL jsonb_array_elements_text(p.data->'children') pc(child) ON TRUE
LEFT   JOIN test c ON c.id = pc.child::int;

db<>fiddle here

About jsonb_array_elements_text():

How to turn JSON array into Postgres array?

Use the -> operator instead of ->> in the reference to children. The way you have it, you'd first cast json / jsonb to text and then back to json.

The clean way to call a set-returning function is LEFT [OUTER] JOIN LATERAL. This includes rows without children. To exclude those, change to a [INNER] JOIN LATERAL or CROSS JOIN - or the shorthand syntax with a comma:

, json_array_elements(p.data->'children') pc(child)

Avoiding duplicate column names in result.

With `json` and `json_array_elements()` in pg 9.3

SELECT p.id AS p_id, p.data AS p_data
     , c.id AS c_id, c.data AS c_data
FROM   test p
LEFT   JOIN LATERAL json_array_elements(p.data->'children') pc(child) ON TRUE
LEFT   JOIN test c ON c.id = pc.child::text::int;

_{Old sqlfiddle}

Aside: A normalized DB design with basic data types would be way more efficient for this.

Kokizzu · Accepted Answer · 2014-11-30 06:59:21Z

Nevermind, I found the way

SELECT *
 FROM ( SELECT *, json_array_elements((data->>'children')::JSON) child FROM test) x1
   LEFT JOIN test x2
    ON x1.child::TEXT::INT = x2.id
;

 id |                 data                 | child | id |               data
----+--------------------------------------+-------+----+-----------------------------------
  1 | {"parent": null, "children": [2, 3]} | 2     |  2 | {"parent": 1, "children": [4, 5]}
  1 | {"parent": null, "children": [2, 3]} | 3     |  3 | {"parent": 1, "children": []}
  2 | {"parent": 1, "children": [4, 5]}    | 4     |  4 | {"parent": 2, "children": []}
  2 | {"parent": 1, "children": [4, 5]}    | 5     |  5 | {"parent": 2, "children": []}

                                                QUERY PLAN                                                 
-----------------------------------------------------------------------------------------------------------
 Hash Left Join  (cost=37.67..4217.38 rows=123000 width=104)
   Hash Cond: ((((json_array_elements(((test.data ->> 'children'::text))::json)))::text)::integer = x2.id)
   ->  Seq Scan on test  (cost=0.00..643.45 rows=123000 width=36)
   ->  Hash  (cost=22.30..22.30 rows=1230 width=36)
         ->  Seq Scan on test x2  (cost=0.00..22.30 rows=1230 width=36)

or

SELECT *
 FROM test x1
    LEFT JOIN ( SELECT *, json_array_elements((data->>'children')::JSON) child FROM test) x2
    ON x1.id = x2.child::TEXT::INT
;

 id |                 data                 | id |                 data                 | child 
----+--------------------------------------+----+--------------------------------------+-------
  2 | {"parent": 1, "children": [4, 5]}    |  1 | {"parent": null, "children": [2, 3]} | 2
  3 | {"parent": 1, "children": []}        |  1 | {"parent": null, "children": [2, 3]} | 3
  4 | {"parent": 2, "children": []}        |  2 | {"parent": 1, "children": [4, 5]}    | 4
  5 | {"parent": 2, "children": []}        |  2 | {"parent": 1, "children": [4, 5]}    | 5
  1 | {"parent": null, "children": [2, 3]} |    |                                      | 

                                                QUERY PLAN                                                 
-----------------------------------------------------------------------------------------------------------
 Hash Right Join  (cost=37.67..4217.38 rows=123000 width=104)
   Hash Cond: ((((json_array_elements(((test.data ->> 'children'::text))::json)))::text)::integer = x1.id)
   ->  Seq Scan on test  (cost=0.00..643.45 rows=123000 width=36)
   ->  Hash  (cost=22.30..22.30 rows=1230 width=36)
         ->  Seq Scan on test x1  (cost=0.00..22.30 rows=1230 width=36)

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