Redshift Postgresql - How to Parse Nested JSON - sql

Question

I am trying to parse a JSON text using JSON_EXTRACT_PATH_TEXT() function.
JSON sample:
{
"data":[
{
"name":"ping",
"idx":0,
"cnt":27,
"min":16,
"max":33,
"avg":24.67,
"dev":5.05
},
{
"name":"late",
"idx":0,
"cnt":27,
"min":8,
"max":17,
"avg":12.59,
"dev":2.63
}
]
}
'
I tried JSON_EXTRACT_PATH_TEXT(event , '{"name":"late"}', 'avg') function to get 'avg' for name = "late", but it returns blank.
Can anyone help, please?
Thanks

Answer 1

This is a rather complicated task in Redshift, that, unlike Postgres, has poor support to manage JSON, and no function to unnest arrays.
Here is one way to do it using a number table; you need to populate the table with incrementing numbers starting at 0, like:
create table nums as
select 0 i union all select 1 union all select 2 union all select 3
union all select 4 union all select 5 n union all select 6
union all select 7 union all select 8 union all select 9
;
Once the table is created, you can use it to walk the JSON array using json_extract_array_element_text(), and check its content with json_extract_path_text():
select json_extract_path_text(item, 'avg') as my_avg
from (
select json_extract_array_element_text(t.items, n.i, true) as item
from (
select json_extract_path_text(mycol, 'data', true ) as items
from mytable
) t
inner join nums n on n.i < json_array_length(t.items, true)
) t
where json_extract_path_text(item, 'name') = 'late';

Answer 2

You'll need to use json_array_elements for that:
select obj->'avg'
from foo f, json_array_elements(f.event->'data') obj
where obj->>'name' = 'late';
Working example
create table foo (id int, event json);
insert into foo values (1,'{
"data":[
{
"name":"ping",
"idx":0,
"cnt":27,
"min":16,
"max":33,
"avg":24.67,
"dev":5.05
},
{
"name":"late",
"idx":0,
"cnt":27,
"min":8,
"max":17,
"avg":12.59,
"dev":2.63
}]}');