How to count setof / number of keys of JSON in postgresql?

Question

I have a column in jsonb storing a map, like {'a':1,'b':2,'c':3} where the number of keys is different in each row.

I want to count it -- jsonb_object_keys can retrieve the keys but it is in setof

Are there something like this?

(select count(jsonb_object_keys(obj) from XXX )

(this won't work as ERROR: set-valued function called in context that cannot accept a set)

Postgres JSON Functions and Operators Document

json_object_keys(json)
jsonb_object_keys(jsonb)

setof text  Returns set of keys in the outermost JSON object.
json_object_keys('{"f1":"abc","f2":{"f3":"a", "f4":"b"}}')  

json_object_keys
------------------
f1
f2

Crosstab isn't feasible as the number of key could be large.

Answer 1

Shortest:

SELECT count(*) FROM jsonb_object_keys('{"a": 1, "b": 2, "c": 3}'::jsonb);

Returns 3

If you want all json number of keys from a table, it gives:

SELECT (SELECT COUNT(*) FROM json_object_keys(myJsonField)) nbr_keys FROM myTable;

Answer 2

You could convert keys to array and use array_length to get this:

select array_length(array_agg(A.key), 1) from (
    select json_object_keys('{"f1":"abc","f2":{"f3":"a", "f4":"b"}}') as key
) A;

If you need to get this for the whole table, you can just group by primary key.