How to cast json array to text array?

Question

This workaround not works

CREATE FUNCTION json_array_castext(json) RETURNS text[] AS $f$
  SELECT array_agg(x::text) FROM json_array_elements($1) t(x);
$f$ L         


        

Answer 1

Oto's answer was a lifesaver, but it did have one boundary case that had me racking my brain. Due to the lossy nature of the cast, it works perfectly except in the case where you've got an empty json array. In that case you would expect an empty array to be returned, but it actually returns nothing. As a workaround, if you just concatenate the return value with an empty array it will have no affect in cases where there is actually a return, but do the right thing when you've got an empty array. Here's the updated SQL functions (for both json and jsonb) that implement the workaround.

CREATE or replace FUNCTION json_array_casttext(json) RETURNS text[] AS $f$
    SELECT array_agg(x) || ARRAY[]::text[] FROM json_array_elements_text($1) t(x);
$f$ LANGUAGE sql IMMUTABLE;

CREATE or replace FUNCTION jsonb_array_casttext(jsonb) RETURNS text[] AS $f$
    SELECT array_agg(x) || ARRAY[]::text[] FROM jsonb_array_elements_text($1) t(x);
$f$ LANGUAGE sql IMMUTABLE;

There are a few peculiarities like this one that point to the rough edges at integrating a document database into a mature relational one, but Postgres does an admirable job at handling most of them.