Remove NULL fields from JSON in Greenplum - json

Question

Using Greenplum 5.* database which is based on Postgres 8.4.
I am using row_to_json and array_to_json functions to create JSON output; but this ending up having keys with null values in JSON. Postgres latest version have json_strip_null function to remove keys with null values.
I need to import the generated JSON files to MongoDB; but mongoimport also doesn't have option to ignore null keys from JSON.
One way I tried it to create JSON file with null and then use sed to remove null fields from JSON file.
sed -i 's/\(\(,*\)"[a-z_]*[0-9]*":null\(,*\)\)*/\3/g' output.json
But looking for a way to do it database itself as it will be faster. Any suggestions how to render json_strip_null function in Greenplum without affecting the query performance?

Answer 1

i've had the same issue in GP 5.17 on pg8.3 - and have had success with this regex to remove the null value key-pairs. i use this in the initial insert to a json column, but you could adapt however:
select
col5,
col6,
regexp_replace(regexp_replace(
(SELECT row_to_json(j) FROM
(SELECT
col1,col2,col3,col4
) AS j)::text,
'(?!{|,)("[^"]+":null[,]*)','','g'),'(,})$','}')::json
AS nvp_json
from foo
working from the inside-out, the result of the row_to_json constructor is first cast to text, then the inner regexp replaces any "name":null, values, the outer regexp trims any hanging commas from the end, and finally the whole thing is cast back to json.

Answer 2

I solved this problem using plpython function. This generic function can be used to remove null and empty valued keys from any JSON.
CREATE OR REPLACE FUNCTION json_strip_null(json_with_nulls json)
RETURNS text
AS $$
import json
def clean_empty(d):
if not isinstance(d, (dict, list)):
return d
if isinstance(d, list):
return [v for v in (clean_empty(v) for v in d) if v not in (None, '')]
return {k: v for k, v in ((k, clean_empty(v)) for k, v in d.items()) if v not in (None, '')}
json_to_dict = json.loads(json_with_nulls)
json_without_nulls = clean_empty(json_to_dict)
return json.dumps(json_without_nulls, separators=(',', ':'))
$$ LANGUAGE plpythonu;
This function can be used as,
SELECT json_strip_null(row_to_json(t))
FROM table t;

Answer 3

You can use COALESCE to replace the nulls with an empty string or another value.
https://www.postgresql.org/docs/8.3/functions-conditional.html
The COALESCE function returns the first of its arguments that is not null. Null is returned only if all arguments are null. It is often used to substitute a default value for null values when data is retrieved for display, for example:
SELECT COALESCE(description, short_description, '(none)') ...
This returns description if it is not null, otherwise short_description if it is not null, otherwise (none).
...