Hi, On 2020-06-25 10:44:42 -0700, Jeff Davis wrote: > There are only two possible paths: HashAgg and Sort+Group, and we need > to pick one. If the planner expects one to spill, it is likely to > expect the other to spill. If one spills in the executor, then the > other is likely to spill, too. (I'm ignoring the case with a lot of > tuples and few groups because that doesn't seem relevant.)
There's also ordered index scan + Group. Which will often be vastly better than Sort+Group, but still slower than HashAgg. > Imagine that there was only one path available to choose. Would you > suggest the same thing, that unexpected spills can exceed work_mem but > expected spills can't? I'm not saying what I propose is perfect, but I've yet to hear a better proposal. Given that there *are* different ways to implement aggregation, and that we use expected costs to choose, I think the assumed costs are relevant. Greetings, Andres Freund
