CREATE JOIN STATISTICS ON t1 JOIN t2 USING (somecol);
That way we let an admin who's tuning queries direct effort at problem areas. It's not automagical, but it's an area where tools could analyze pg_stat_statements to direct effort, much like is currently done for index creation. Like index creation I don't think it's practical to do this entirely automatically and behind the scenes since collecting the stats for all possibilities rapidly gets prohibitive.
There's an extremely common case that could (and IMO should) be automated though, which is computing these statistics for all foreign keys. We can have a way to disable that for specific keys if necessary, but I'd bet it's extremely rare to have a FK that you never join on.
