How to flatten out nested array of strings in json column? - sql

Question

I have the following table:
id
contents
123
{ blocks: [{ text: "abc" }, { text: "123" }] }
foo
{ blocks: [{ text: "bar" }, { text: "moretext" }, { text: "ok" }] }
I want to write a view of the above that looks like:
id
contents
raw_text
123
{blocks: [{text: "abc"}, {text: "123"}]}
abc, 123
foo
{blocks: [{text: "bar"}, {text: "moretext"}, { text: "ok"}]}
bar, moretext, ok
This was the query I tried running:
select post.id, array_to_string(array_agg(jsonb_array_elements(post.contents -> 'blocks') ->> 'text')) as paragraphs from post group by id
But it results in the error
aggregate function calls cannot contain set-returning function calls.

Answer 1

If a JSON array of all the values is also acceptable, you can use a JSON path query:
select id, contents,
jsonb_path_query_array(contents, '$.blocks[*].text')
from post;
As there is no simply cast from a JSON array to a native Postgres array, and you do need that as a CSV string, you need to unnest and aggregate with a scalar sub-query:
select id, contents,
(select string_agg(x.item ->> 'text', ', ')
from jsonb_array_elements(contents -> 'blocks') as x(item)) as raw_text
from post;
The reason for your error is, that you are mixing nesting multiple aggregate and set returning function which simply isn't supported.