Postgres "unnesting" multiple json arrays simultaneouesly with different depths

Question

I am trying to explode/unnest a row of jsonb arrays into individual rows and keep any rows that would result in nulls.

Fiddle: https://www.db-fiddle.com/f/pgd6E5yKf8NzPtgboeoC6c/7

Input table: (I am stuck with the input being in this form)

id	name	color	price	description
1	["Banana"]	[["Green"]]	[[[1]]]	[[["Good"]]]
2	["Banana"]	[["Yellow"]]	[[[]]]	[[[]]]
3	["Apple","Carrot"]	[["Red"],["Orange"]]	[[[10]],[[14]]]	[[["Round"]],[["Long"]]]
4	["Grape","Pineapple"]	[["Green"],["Brown","Pink"]]	[[[2]],[[13],[29]]]	[[["Small"]],[["Short"],["Tall"]]]
5	["Orange","Potato"]	[["Orange"],["Purple","Brown"]]	[[[2]],[[9],[]]]	[[["Bright"]],[["Gross"],[]]]

Desired Output:

id	name	color	price	description
1	Banana	Green	1	Good
2	Banana	Yellow	null	null
3	Apple	Red	10	Round
3	Carrot	Orange	14	Long
4	Grape	Green	2	Small
4	Pineapple	Brown	13	Short
4	Pineapple	Pink	29	Tall
5	Orange	Orange	2	Bright
5	Potato	Purple	9	Gross
5	Potato	Brown	null	null

I can get the output I want with this query, but I am trying to generate these queries using a templating engine and I'm trying to account for any number of elements being any depth of nested arrays. Trying to template this to handle that will be less than ideal.

-- Gives expected results
WITH first_level AS (
  SELECT
      id,
      jsonb_array_elements(name) AS name,
      jsonb_array_elements(color) AS color,
      jsonb_array_elements(price) AS price,
      jsonb_array_elements(description) AS description
  FROM food
),
second_level AS (
SELECT
      id,
      name,
      jsonb_array_elements(color) AS color,
      jsonb_array_elements(price) AS price,
      jsonb_array_elements(description) AS description
  FROM first_level
  )
SELECT
      id,
      name,
      color,
      CASE
            WHEN price = '[]'::jsonb THEN null
            ELSE jsonb_array_elements(price)
      END AS price,
      CASE
            WHEN description = '[]'::jsonb THEN null
            ELSE jsonb_array_elements(description)
      END AS description
FROM second_level

I've tried unnesting them all at once but with pg 9.6 that gives back a bad result and doesnt have the nulls, It works better on pg10+ but it still drops the rows with empty arrays.

Is there a cleaner way to achieve this result? I've run across examples of using lateral joins but none of them are exact matches and I cant figure them out to even get a result close to what I need.

Thanks

Answer 1

Unnesting multiple set-returning functions in parallel behaved in mysterious ways (borderline broken) in Postgres 9.6. That got fixed with Postgres 10. See:

• What is the expected behaviour for multiple set-returning functions in SELECT clause?

The same fix also made set-returning functions in a CASE construct illegal. Your query only works on account of that superseded behaviour in Postgres 9.6 and breaks in later versions. Same as my simpler rCTE solution:

db<>fiddle here

You didn't specify what's allowed in your input format. Making some assumptions, this works in modern Postgres (and in Postgres 9.6, too):

Custom function (accepts non-arrays, too):

CREATE OR REPLACE FUNCTION f_jsonb_array_elements_if_any(jsonb)
  RETURNS SETOF jsonb
  LANGUAGE plpgsql IMMUTABLE STRICT PARALLEL SAFE AS
$func$
BEGIN
   IF jsonb_typeof($1) = 'array' THEN 
      IF $1 = '[]' THEN
         RETURN NEXT jsonb 'null';
      ELSE
         RETURN QUERY SELECT jsonb_array_elements($1);
      END IF;
   ELSE
      RETURN NEXT $1;
   END IF;
END
$func$;

Main query (with multiple advanced SQL features):

WITH RECURSIVE cte AS (
   SELECT id, 1 AS lvl, ARRAY[ord] AS path, t.*
   FROM   food
   LEFT   JOIN LATERAL ROWS FROM (
      f_jsonb_array_elements_if_any(name)
    , f_jsonb_array_elements_if_any(color)
    , f_jsonb_array_elements_if_any(price)
    , f_jsonb_array_elements_if_any(description)
      ) WITH ORDINALITY t(n, c, p, d, ord) ON true

   UNION ALL
   SELECT id, lvl + 1, path || t.ord, t.*
   FROM   cte c
   LEFT   JOIN LATERAL ROWS FROM (
      f_jsonb_array_elements_if_any(n)
    , f_jsonb_array_elements_if_any(c)
    , f_jsonb_array_elements_if_any(p)
    , f_jsonb_array_elements_if_any(d)
      ) WITH ORDINALITY t(n, c, p, d, ord) ON true
   WHERE  'array' IN (jsonb_typeof(c.n), jsonb_typeof(c.c), jsonb_typeof(c.p), jsonb_typeof(c.d))
   )
SELECT id, (first_value(n) OVER (PARTITION BY id, grp ORDER BY path))->>0 AS name
     , c->>0 AS color, p->>0 AS price, d->>0 AS description
FROM  (
   SELECT count(n) OVER (PARTITION BY id ORDER BY path) AS grp, *
   FROM   cte c
   WHERE  lvl = (SELECT max(lvl) FROM cte)
   ) sub;

db<>fiddle here

Cycle with a recursive CTE, unnesting arrays until no array is left.

In the outer SELECT only use rows from the lat iteration (max(lvl))

Where a single element in name was combined with multiple in the other columns, we get genuine null values (as opposed to a jsonb 'null' from an empty array). Fill in the last actual value for the same group of rows.

Finally extract text from the jsonb wrapper.

This is one of the rare occasions where a ROWS FROM construct is instrumental. See:

• Unnest multiple arrays in parallel
• How to "zip" multiple nested JSON arrays without using id key?