Add support JSONArrayAgg

[Robert Silén]

I suggest to add ArrayAgg and related (aggregation) functions for MariaDB to Django. It has been noted in MariaDB Foundation ticket: ​https://jira.mariadb.org/browse/MDBF-690

E.g. ArrayAgg is mentioned on PostgreSQL specific aggregation functions:
​https://docs.djangoproject.com/en/dev/ref/contrib/postgres/aggregates/#arrayagg

The equivalent in MariaDB is presumably this:
​https://mariadb.com/kb/en/json_arrayagg/

More equivalent MariaDB aggregate functions are listed at the following address.
​https://mariadb.com/kb/en/aggregate-functions/
Some have same name 1:1. I'm guessing e.g. StringAgg could be done with GROUP_CONCAT.

Feel free to ask more here, or in the MariaDB Foundation ticket.

[Simon Charette]

The contrib.postgres.ArrayAgg expression you pointed at uses Postgres arrays and MariaDB doesn't have native array support so they are not equivalent.

JSON_ARRAYAGG is a standard SQL function implemented on Postgres, SQLite, MySQL, SQLite, Oracle, and MariaDB so if we were to include it in core it should be for all supported backends and not only for MariaDB.

It should be trivial to implement by subclassing Aggregate in fact I suspect that the following should work on most backends by default

class JSONArrayAgg(Aggregate):
    function = "JSON_ARRAYAGG"
    output_field = JSONField()
    arity = 1

Or with subtle adjusted as using json_group_array on SQLite.

As for StringAgg with a fallback on GROUP_CONCAT on MySQL and MariaDB it is already being explored in #35444 which plans to add support for configurable support for ORDER BY in aggregates.

[Lufafa Joshua]

​Draft PR