Return elements of Redshift JSON array on separate rows

Thanks to this inspired blog post, I’ve been able to craft a solution. This is:

1. Create a look-up table to effectively ‘iterate’ over the elements of each array. The number of rows in this table has be equal to or greater than the maximum number of elements of arrays. Let’s say this is 4 (it can be calculated using SELECT MAX(JSON_ARRAY_LENGTH(metadata)) FROM input_table):

   CREATE VIEW seq_0_to_3 AS
       SELECT 0 AS i UNION ALL                                      
       SELECT 1 UNION ALL
       SELECT 2 UNION ALL    
       SELECT 3          
   );
   

2. From this, we can create one row per JSON element:

   WITH exploded_array AS (                                                                          
       SELECT id, JSON_EXTRACT_ARRAY_ELEMENT_TEXT(metadata, seq.i) AS json
       FROM input_table, seq_0_to_3 AS seq
       WHERE seq.i < JSON_ARRAY_LENGTH(metadata)
     )
   SELECT *
   FROM exploded_array;
   

   Producing:

    id | json
   ------------------------------
    1  | {"pet":"dog"}
    1  | {"country":"uk"}
    2  | {"pet":"cat"}
    4  | {"country":"germany"}
    4  | {"education":"masters"}
    4  | {"country":"belgium"}
   

3. However, I was needing to extract the field names/values. As I can’t see any way to extract JSON field names using Redshift’s limited functions, I’ll do this using a regular expression:

   WITH exploded_array AS (                                                                                       
       SELECT id, JSON_EXTRACT_ARRAY_ELEMENT_TEXT(metadata, seq.i) AS json
       FROM input_table, seq_0_to_3 AS seq
       WHERE seq.i < JSON_ARRAY_LENGTH(metadata)
   )
   SELECT id, field, JSON_EXTRACT_PATH_TEXT(json, field)
   FROM (
       SELECT id, json, REGEXP_SUBSTR(json, '[^{"]\\w+[^"]') AS field
       FROM exploded_array
   );