How to cross join unnest a JSON array in Presto

Question

Given a table that contains a column of JSON like this:

{"payload":[{"type":"b","value":"9"}, {"type":"a","value":"8"}]}
{"payload":[{"type":"c","value":"7"}, {"type":"b","value":"3"}]}

How can I write a Presto query to give me the average b value across all entries?

So far I think I need to use something like Hive's lateral view explode, whose equivalent is cross join unnest in Presto.

But I'm stuck on how to write the Presto query for cross join unnest.

How can I use cross join unnest to expand all array elements and select them?

Answer 1

Here's an example of that

with example(message) as (
VALUES
(json '{"payload":[{"type":"b","value":"9"},{"type":"a","value":"8"}]}'),
(json '{"payload":[{"type":"c","value":"7"}, {"type":"b","value":"3"}]}')
)


SELECT
        n.type,
        avg(n.value)
FROM example
CROSS JOIN
    UNNEST(
            CAST(
                JSON_EXTRACT(message,'$.payload')
                    as ARRAY(ROW(type VARCHAR, value INTEGER))
                    )
                ) as x(n)
WHERE n.type = 'b'
GROUP BY n.type

with defines a common table expression (CTE) named example with a column aliased as message

VALUES returns a verbatim table rowset

UNNEST is taking an array within a column of a single row and returning the elements of the array as multiple rows.

CAST is changing the JSON type into an ARRAY type that is required for UNNEST. It could easily have been an ARRAY<MAP< but I find ARRAY(ROW( nicer as you can specify column names, and use dot notation in the select clause.

JSON_EXTRACT is using a jsonPath expression to return the array value of the payload key

avg() and group by should be familiar SQL.

Answer 2

As you pointed out, this was finally implemented in Presto 0.79. :)

Here is an example of the syntax for the cast from here:

select cast(cast ('[1,2,3]' as json) as array<bigint>);

Special word of advice, there is no 'string' type in Presto like there is in Hive. That means if your array contains strings make sure you use type 'varchar' otherwise you get an error msg saying 'type array does not exist' which can be misleading.

select cast(cast ('["1","2","3"]' as json) as array<varchar>);

Answer 3

The problem was that I was running an old version of Presto.

unnest was added in version 0.79

https://github.com/facebook/presto/blob/50081273a9e8c4d7b9d851425211c71bfaf8a34e/presto-docs/src/main/sphinx/release/release-0.79.rst