Planner max time limit

The plan for this kind of query takes an indefinite amount of time:

SELECT label, AVG(foo) FROM (
  SELECT label, AVG(x) as foo FROM table1 GROUP BY label
  UNION ALL
  SELECT label, AVG(y) as foo FROM table2 GROUP BY label
  UNION ALL
  ... 30 times
) GROUP BY label

That is, the query is not executed at all, because the planning does not finish (we're talking about hours).

By disabling planner.enable_multiphase_agg, the planning completes immediately and the query runs.
But I guess this can happen also for other kind of queries.

I'd like to suggest a way to limit the planning time. Once this time elapses, the planner outputs the best plan found so far.