Stored procedure update query in postgres

Question

I have data in table which is of type text so i am converting it to json. Based on id of data i want to update a value of column.

SELECT
  CASE
    WHEN is_json(data)
      THEN data::json #> '{id}'
    ELSE
      NULL
  END
FROM table;

UPDATE table SET hash = md5(
result from above query??
);

Some kind of:

How can i write the update query?

SELECT 
  CASE
    WHEN is_json(data)
      THEN data::json #> md5('{id}')
    ELSE
      NULL
  END
FROM table;

I hava json stored in a text column. data:

{'id': 'e807b554-fd15-4f7c-bbc5-d9531c8174a9','array': ['a'], 'rank': 151}

My stored procedure is:

CREATE OR REPLACE FUNCTION is_json(input_text varchar) RETURNS boolean AS $$
  DECLARE
    maybe_json json;
  BEGIN
    BEGIN
      maybe_json := input_text;
    EXCEPTION WHEN others THEN
      RETURN FALSE;
    END;

    RETURN TRUE;
  END;
$$ LANGUAGE plpgsql IMMUTABLE;

When i select the query:

select * from table where is_json(data);

I get empty row. Whereas data has json in form of text.

Answer 1

Is this what you want?

update mytable 
set hash = md5(case when is_json(data) then data::json #>> '{id}' end)

Or if you want to only update rows that contain valid json:

update mytable 
set hash = md5(data::json #>> '{id}')
where is_json(data)