[davidrowley]

> you don't want your "optimizer stats" to be as big as the whole dataset itself. So optimizer has limited information, by design, and it has to come up with _something_ in a very short amount of time.

This is very true. PostgreSQL does not do any proactive plan caching, so it's important that the planner remains fast. It is possible to adjust the number of stats targets to control the size of the histograms and most common values list. Upping that can be useful for OLAP-type workloads.

> I imagine that in addition to table/column stats, is also uses structural info as its inputs, like existence of (enabled & valid) constraints for example.

Yes. Foreign key constraints are used to assist with join selectivity estimations. PG17 (when released) should be able to make more use of NOT NULL constraints to improve plans.