Cast polygon to json array and json array to polygon

Question

I've created this two functions, and I want to know if there is a better way to cast json arrays to polygons without having to install the PostGIS extension.

Raw type:

((-34.888733,-57.956764),(-34.92303,-57.99367),(-34.953579,-57.95255))

polygon_to_json:

["-34.888733,-57.956764","-34.92303,-57.99367","-34.953579,-57.95255"]

json_to_polygon:

((-34.888733,-57.956764),(-34.92303,-57.99367),(-34.953579,-57.95255))

create or replace function json_to_polygon(p_poly json, out p_out polygon) returns polygon
parallel safe
returns null on null input
immutable
language plpgsql
as $$
begin
    if(p_poly is null) then
        return;
    end if;

    select
        polygon(concat('((', string_agg(x, '),('), '))'))
    from
        json_array_elements_text(p_poly) x
    into
        p_out;
end;
$$;


create or replace function polygon_to_json(p_poly polygon) returns json
parallel safe
returns null on null input
immutable
language plpgsql
as $$
begin
    if(p_poly is null) then
        return null;
    end if;

    return to_json(string_to_array(replace(replace(p_poly::text, '((', ''), '))', ''), '),('));
end;
$$;

Erwin Brandstetter · Accepted Answer · 2024-08-16 02:54:17Z

Yes, there is a better way:

CREATE OR REPLACE FUNCTION json_to_polygon (p_poly json)
  RETURNS polygon
  LANGUAGE sql PARALLEL SAFE STRICT IMMUTABLE
RETURN replace(replace(replace(
            p_poly::text
         , '","', '),(')
         , '["', '((')
         , '"]', '))')::polygon;


CREATE OR REPLACE FUNCTION polygon_to_json (p_poly polygon)
  RETURNS json
  LANGUAGE sql PARALLEL SAFE STRICT IMMUTABLE
RETURN replace(replace(replace(
            p_poly::text
         , '),(', '","')
         , '((' , '["')
         , '))' , '"]')::json;

fiddle

I use the syntax variant for SQL-standard functions (added with Postgres 14. See:
- What does BEGIN ATOMIC mean in a PostgreSQL SQL function / procedure?
Obviously, since both functions are declared STRICT ( = RETURNS NULL ON NULL INPUT), we need not check for null input.
The simple functions can be LANGUAGE sql, so they can be inlined. See:
- How can I find out if a SQL function can be inlined?
concat() is only STABLE, not IMMUTABLE, use the IMMUTABLE concatenation operator || instead. Then declaring the function IMMUTABLE is correct, and also won't stand in the way of function inlining. See:
- How to concatenate columns in a Postgres SELECT?
- Combine two columns and add into one new column
A 3rd replace() should be faster than pulling in string_to_array().
Finally, I make json_to_polygon() the exact reverse of polygon_to_json(), which should again be faster, and easier to read and maintain.

All that said, PostGIS does have some functions to help with that ...

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