SELECT C.table_schema, C.table_name, 
Round((SELECT Count(*) 
FROM information_schema.Columns AS C1 
WHERE C1.is_nullable='YES' AND C1.table_schema=C.table_schema AND C1.table_name=C.table_name)::decimal*100/Count(C.column_name)::decimal,2) AS percentage_of_optional
FROM information_schema.columns C INNER JOIN information_schema.tables T USING (table_schema, table_name)
INNER JOIN information_schema.schemata S ON T.table_schema = S.schema_name
WHERE T.table_type = 'BASE TABLE' 
AND (T.table_schema = 'public' OR S.schema_owner<>'postgres')
GROUP BY C.table_schema, C.table_name
ORDER BY percentage_of_optional DESC, C.table_schema, C.table_name;