How to apply a function to each element of a json array?

Question

Given this function:

CREATE OR REPLACE FUNCTION extract_key(
  i_m JSONB,
  i_key TEXT
) RETURNS JSONB AS $$
BEGIN
  RETURN i_m -> i_key;
END;
$$ LANGUAGE 'plpgsql';

I'd like to write a function extract_key_array where:

SELECT extract_key_array('hello', '[{"hello": {"id": 1}} ,{"hello": {"id": 2}}]')

outputs a json array: '[{"id": 1},{"id": 2}]'

Answer 1

Your answer is basically good. A couple of improvments:

CREATE OR REPLACE FUNCTION extract_key_array(_key text, _arr jsonb)
  RETURNS jsonb
  LANGUAGE sql STABLE PARALLEL SAFE AS
$func$
SELECT jsonb_agg(elem -> _key) FROM jsonb_array_elements(_arr) elem;
$func$;

Works with your desired function call:

SELECT extract_key_array('hello', '[{"hello": {"id": 1}} ,{"hello": {"id": 2}}]')

db<>fiddle here

• A simple SQL function does the job and can be inlined.

• Declare the function STABLE and PARALLEL SAFE (because it is!) This allows for various optimizations in query plans.
  (Don't declare it IMMUTABLE as the contained jsonb_agg() is only STABLE for some unknown reason.)

• You answer switched the order of function parameters displayed in the question.

About function inlining:

• Function executes faster without STRICT modifier?

Answer 2

This works but maybe there's a faster method.

CREATE OR REPLACE FUNCTION extract_key_array(
  i_arr JSONB,
  i_key TEXT
) RETURNS JSONB AS $$
BEGIN
  RETURN jsonb_agg(extract_key(out,i_key)) FROM jsonb_array_elements(i_arr) AS out;
END;
$$ LANGUAGE 'plpgsql';