Presto Produce JSON results - sql

Question

I have a json table which was created by
CREATE TABLE `normaldata_source`(
`column1` int,
`column2` string,
`column3` struct<column4:string>)
A sample data is:
{
"column1": 9,
"column2": "Z",
"column3": {
"column4": "Y"
}
}
If I do
SELECT column3
FROM normaldata_source
it will produce a result {column4=y}. However, I want it to be in json form {"column4": "y"}
Is this possible?
*Edit This query gives me the following result:
SELECT CAST(column3 AS JSON) as column3_json
FROM normaldata_source

Answer 1

As of Trino 357 (formerly known as Presto SQL), you can now cast a row to JSON and it will preserve the column names:
WITH normaldata_source(column1, column2, column3) AS (
VALUES (9, 'Z', cast(row('Y') as row(column4 varchar)))
)
SELECT cast(column3 as json)
FROM normaldata_source
=>
_col0
-----------------
{"column4":"Y"}
(1 row)

Answer 2

I encountered this same problem and was thoroughly stumped on how to proceed in light of deep compositional nesting/structs. I'm using Athena (managed Presto w/ Hive Connector from AWS). In the end, I worked around it by doing a CTAS (create table as select) where I selected the complex column I wanted, under the conditions I wanted) and wrote it to an external table with an underlying SerDe format of JSON. Then, via the HiveConnector's $path magic column (or by listing the files under the external table location), I obtain the resulting files and streamed out of those.
I know this isn't a direct answer to the question at hand - I believe we have to wait for https://github.com/trinodb/trino/pull/3613 in order to support arbitrary struct/array compositions -> json. But maybe this will help someone else who kind of assumed they'd be able to do this.
Although I originally saw this as an annoying workaround, I'm now starting to think it was the right call for my application anyway