On Wed, 16 Jun 2021 at 15:08, Peter Geoghegan <p...@bowt.ie> wrote: > It seems important to distinguish between risk and uncertainty -- > they're rather different things. The short version is that you can > model risk but you cannot model uncertainty. This seems like a problem > of uncertainty to me.
You might be right there. "Uncertainty" or "Certainty" seems more like a value that clauselist_selectivity() would be able to calculate itself. It would just be up to the planner to determine what to do with it. One thing I thought about is that if the costing modal was able to separate out a cost of additional (unexpected) rows then it would be easier for add_path() to take into account how bad things might go if we underestimate. For example, in an unparameterized Nested Loop that estimates the outer Path to have 1 row will cost an entire additional inner cost if there are 2 rows. With Hash Join the cost is just an additional hashtable lookup, which is dead cheap. I don't know exactly how add_path() would weigh all that up, but it seems to me that I wouldn't take the risk unless I was 100% certain that the Nested Loop's outer Path would only return 1 row exactly, if there was any chance at all it could return more, I'd be picking some other join method. David
