PostgreSql : Json Array to Rows using Lateral Join

Question

I have two following JSON Array in details field of my table and need to evaluate the query as I use in another relational table.

{
    "city": "London",
    "name": "Sainburry",
    "quantities": [112, 145, 222, 122, 124],
    "prices": [4, 4, 4, 0, 3],
    "dates": ["13.05.2020", "14.05.2020", "15.05.2020", "16.05.2020", "17.05.2020"]
}

I want to evaluate the following query for this JSON Array:

select quantities,
       prices,
       AVG(quantities/prices::float) as ratio
from my_table
where city = 'London'
group by quantities, prices;

I used the following query and many similar queries including lateral join:

select q.*
from my_table mt
  cross join json_array_elements_text(details -> 'quantities') as q

But, when adding the other fields (prices and dates) to the query by cross join, the rows multiplied. So, I am looking for a new feature Lateral Join to use, but not able to apply properly. How can I obtain the result I obtained previous query by using Lateral Join in PostgreSQL? Any help would be appreciated.

Update:

Here is the fiddle. I can evaluate the desired result if I succeed to convert the json array values to rows without multiplying (5 records should be returned). Just help me to convert json array to row using lateral join and json_array_elements_text.

Answer 1

Seems you need WITH ORDINALITY along with LEFT JOIN LATERALs to match the corresponding elements of the arrays due to the order in the arrays, respectively :

SELECT q.elm AS quantities, p.elm AS prices, 
       AVG(p.elm::float/q.elm::float) AS ratio
  FROM my_table t0
  LEFT JOIN LATERAL jsonb_array_elements(details -> 'quantities') 
    WITH ORDINALITY AS q(elm, i) ON TRUE
  LEFT JOIN LATERAL jsonb_array_elements(details -> 'prices') 
    WITH ORDINALITY AS p(elm, i) ON q.i = p.i
  LEFT JOIN LATERAL jsonb_array_elements(details -> 'dates') 
    WITH ORDINALITY AS d(elm, i) ON d.i = q.i
 WHERE t0.details ->> 'city' = 'London'   
 GROUP BY q.elm, p.elm;

Demo