Allow read only CTEs in query reports

Description of the issue

There is a check in frappe/core/doctype/report/report.py to make sure, the query starts with a SELECT statement. My query looks like this and is therefore not accepted:

WITH recursive Calendar AS (
   SELECT '2022-03-01' AS CalendarDate
   UNION ALL
   SELECT DATE_ADD(CalendarDate, INTERVAL 1 DAY)
   FROM   Calendar
   WHERE  CalendarDate < '2022-03-10'
)
SELECT c.CalendarDate
       , SUM( IF(t.modified > c.CalendarDate OR t.status = 'To Deliver and Bill', t.total, 0) ) AS TotalValue
FROM   Calendar c INNER JOIN `tabSales Order` t
            ON t.creation <= c.CalendarDate
            AND (t.modified >= c.CalendarDate OR t.status = 'To Deliver and Bill')         
GROUP BY c.CalendarDate 
ORDER BY c.CalendarDate
;

Context information (for bug reports)

Output of bench version

v12.22.0

Observed result

frappe.exceptions.ValidationError: Query must be a SELECT

Expected result

Execution of the query report.

Stacktrace / full error message

Traceback (most recent call last):
  File "apps/frappe/frappe/app.py", line 68, in application
    response = frappe.api.handle()
  File "apps/frappe/frappe/api.py", line 55, in handle
    return frappe.handler.handle()
  File "apps/frappe/frappe/handler.py", line 31, in handle
    data = execute_cmd(cmd)
  File "apps/frappe/frappe/handler.py", line 68, in execute_cmd
    return frappe.call(method, **frappe.form_dict)
  File "apps/frappe/frappe/__init__.py", line 1213, in call
    return fn(*args, **newargs)
  File "apps/frappe/frappe/__init__.py", line 629, in wrapper_fn
    retval = fn(*args, **get_newargs(fn, kwargs))
  File "apps/frappe/frappe/desk/query_report.py", line 244, in run
    result = generate_report_result(report, filters, user, custom_columns, is_tree, parent_field)
  File "apps/frappe/frappe/__init__.py", line 629, in wrapper_fn
    retval = fn(*args, **get_newargs(fn, kwargs))
  File "apps/frappe/frappe/desk/query_report.py", line 86, in generate_report_result
    res = get_report_result(report, filters) or []
  File "apps/frappe/frappe/desk/query_report.py", line 67, in get_report_result
    res = report.execute_query_report(filters)
  File "apps/frappe/frappe/core/doctype/report/report.py", line 117, in execute_query_report
    frappe.throw(_("Query must be a SELECT"), title=_('Report Document Error'))
  File "apps/frappe/frappe/__init__.py", line 444, in throw
    msgprint(msg, raise_exception=exc, title=title, indicator='red', is_minimizable=is_minimizable, wide=wide, as_list=as_list)
  File "apps/frappe/frappe/__init__.py", line 423, in msgprint
    _raise_exception()
  File "apps/frappe/frappe/__init__.py", line 378, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.ValidationError: Query must be a SELECT

[carstenblt]

Also, I believe MariaDB only supports WITH with SELECT: https://mariadb.com/kb/en/with/ (I just tried to work around it with a helper table expecting WITH … AS … INSERT INTO to work, which doesn’t.) So I guess enabling it for MariaDB would be fine.

[carstenblt]

Postgres, and not writing to a table, but an example I recently used to reset auto increment values:

select SETVAL(pg_get_serial_sequence('public.table', 'id'), (SELECT MAX(id) FROM public.table));

At least there you can execute a SETVAL().