I wrote:
> After a bit of further fooling, I found that we could make that
> work with LEFT JOIN LATERAL. This formulation has a different
> problem, which is that if you do want most or all of the output,
> computing each sub-aggregation separately is probably less
> efficient than it could be. But this is probably the better way
> to go unless someone has an even better idea.
Hearing no better ideas, pushed.
regards, tom lane