Postgres find in jsonb nested array - postgresql

Question

I have a case when my data in in nested arrays of jsonb in order to find the value I have to do multiple JSONB_ARRAY_ELEMENTS which is costly and takes a lots of nested code.
The json file has the continents inside countries and inside cities.
I need to access a city value.
Is there a way to make this query simpler and faster?
I was trying to solve it using JSON_EXTRACT_PATH but in order to get in to a array but I need the indexes.
WITH mydata AS (
SELECT '
{
"continents":[
{
"name":"America",
"area":43316000,
"countries":[
{
"country_name":"Canada",
"capital":"Toronto",
"cities":[
{
"city_name":"Ontario",
"population":2393933
},
{
"city_name":"Quebec",
"population":12332
}
]
},
{
"country_name":"Brazil",
"capital":"Brasilia",
"cities":[
{
"city_name":"Sao Paolo",
"population":34534534
},
{
"city_name":"Rio",
"population":445345
}
]
}
]
},
{
"name":"Europa",
"area":10530751,
"countries":[
{
"country_name":"Switzerland",
"capital":"Zurich",
"cities":[
{
"city_name":"Ginebra",
"population":4564565
},
{
"city_name":"Basilea",
"population":4564533
}
]
},
{
"country_name":"Norway",
"capital":"Oslo",
"cities":[
{
"city_name":"Oslo",
"population":3243534
},
{
"city_name":"Steinkjer",
"population":4565465
}
]
}
]
}
]
}
'::JSONB AS data_column
)
SELECT cit.city->>'city_name' AS city,
(cit.city->>'population')::INTEGER AS population
FROM (SELECT JSONB_ARRAY_ELEMENTS(coun.country->'cities') AS city
FROM (SELECT JSONB_ARRAY_ELEMENTS(cont.continent->'countries') AS country
FROM (SELECT JSONB_ARRAY_ELEMENTS(data_column->'continents') AS continent
FROM mydata
) AS cont
WHERE cont.continent #> '{"name":"Europa"}'
) AS coun
WHERE coun.country #> '{"country_name" : "Norway"}'
) AS cit
WHERE cit.city #> '{"city_name": "Oslo"}'
See my nested queries? looks ugly, I can get the answer using: JSONB_EXTRACT_PATH( data_column->'continents', '1', 'countries', '1', 'cities', '0', 'population') but I had to hardcode the array indexes.
Hope you can help me out.
Thanks.

Answer 1

You don't need any nesting, you can do lateral queries:
SELECT
city->>'city_name' AS city,
(city->>'population')::INTEGER AS population
FROM
mydata,
JSONB_ARRAY_ELEMENTS(data_column->'continents') AS continent,
JSONB_ARRAY_ELEMENTS(continent->'countries') AS country,
JSONB_ARRAY_ELEMENTS(country->'cities') AS city
WHERE continent ->> 'name' = 'Europa'
AND country ->> 'country_name' = 'Norway'
AND city ->> 'city_name' = 'Oslo';
(online demo)
However, since you mentioned paths and having to specify indices in there, this is actually the perfect use case for Postgres 12 JSON paths:
SELECT jsonb_path_query(data_column, '$.continents[*]?(#.name == "Europa").countries[*]?(#.country_name=="Norway").cities[*]?(#.city_name=="Oslo")') FROM mydata
(online demo)