MariaDB's `JSON_LENGTH` - alternative for PostgreSQL - sql

Question

I need to convert some queries written for MariaDB to PostgreSQL syntax. Unfortunately they use the JSON_LENGTH function of MariaDB and im struggling to find an alternative with PostgreSQL.
For clarification:
JSON_LENGTH counts the number of 'entries' on the root level of a JSON object/array in MariaDB:
SELECT
JSON_LENGTH('{"test": 123}') as test1, -- 1
JSON_LENGTH('"123"') as test2, -- 1
JSON_LENGTH('123') as test3, -- 1
JSON_LENGTH('[]') as test4, -- 0
JSON_LENGTH('[123]') as test5, -- 1
JSON_LENGTH('[123, 456]') as test6, -- 2
JSON_LENGTH('[123, {"test": 123}]') as test7; -- 2
Partial solutions in PostgreSQL I came up with:
select
json_array_length('[]'::json) as test1, -- 0
json_array_length('["a"]'::json) as test2, -- 1
length(json_object_keys('{"a": "b"}'::json)) as test3; -- 1
json_array_length is not allowed for JSON objects
json_object_keys is not allowed for JSON arrays
Unfortunately I can't manage to combine the two methods I figured out for PostgreSQL:
I tried to use CASE WHEN:
select
case
when json_typeof('["a"]'::json) = 'array' then json_array_length('["a"]'::json)
when json_typeof('["a"]'::json) = 'object' then length(json_object_keys('["a"]'::json))
else 0
end;
Error:
[0A000] ERROR: set-returning functions are not allowed in CASE
-> json_object_keys is the bad guy here
I tried IF ELSE:
select
IF ('array' = json_typeof('["a"]'::json)) THEN json_array_length('["a"]'::json)
ELEIF (json_typeof('["a"]'::json) = 'object') THEN length(json_object_keys('["a"]'::json))
ELSE 0;
Error:
[42601] ERROR: syntax error at or near "THEN"
I can imagine I have an error in my IF ELSE statement, but I'm unable to figure it out.
Is there any way to replicate MariaDB's JSON_LENGTH behavior with PostgreSQL?

Answer 1

You need to count the number of rows returned by jsonb_object_keys():
This:
with data (input) as (
values
('{"test": 123}'::jsonb),
('"123"'),
('123'),
('[]'),
('[123]'),
('[123, 456]'),
('[123, {"test": 123}]')
)
select input,
case jsonb_typeof(input)
when 'array' then jsonb_array_length(input)
when 'object' then (select count(*) from jsonb_object_keys(input))
when 'string' then 1
else 0
end
from data
returns:
input | case
---------------------+-----
{"test": 123} | 1
"123" | 1
123 | 0
[] | 0
[123] | 1
[123, 456] | 2
[123, {"test": 123}] | 2
Of course this can easily be put into a function:
create or replace function my_json_length(p_input jsonb)
returns int
as
$$
select case jsonb_typeof(p_input)
when 'array' then jsonb_array_length(p_input)
when 'object' then (select count(*) from jsonb_object_keys(p_input))
when 'string' then 1
else 0
end;
$$
language sql
immutable;
I would not name it json_length() to avoid any clashes in case Postgres decides to implement such a function in the future (although I am not aware of such a function in the SQL standard).
Note that jsonb is the recommended data type to store JSON values in Postgres.