On Wed, Aug 22, 2018 at 6:43 PM, Andres Freund <and...@anarazel.de> wrote: > Now you can say that'd be solved by bumping the cost up, sure. But > obviously the row / cost model is pretty much out of whack here, I don't > see how we can make reasonable decisions in a trivial query that has a > misestimation by five orders of magnitude.
Before JIT, it didn't matter whether the costing was wrong, provided that the path with the lowest cost was the cheapest path (or at least close enough to the cheapest path not to bother anyone). Now it does. If the intended path is chosen but the costing is higher than it should be, JIT will erroneously activate. If you had designed this in such a way that we added separate paths for the JIT and non-JIT versions and the JIT version had a bigger startup cost but a reduced runtime cost, then you probably would not have run into this issue, or at least not to the same degree. But as it is, JIT activates when the plan looks expensive, regardless of whether activating JIT will do anything to make it cheaper. As a blindingly obvious example, turning on JIT to mitigate the effects of disable_cost is senseless, but as you point out, that's exactly what happens right now. I'd guess that, as you read this, you're thinking, well, but if I'd added JIT and non-JIT paths for every option, it would have doubled the number of paths, and that would have slowed the planner down way too much. That's certainly true, but my point is just that the problem is probably not as simple as "the defaults are too low". I think the problem is more fundamentally that the model you've chosen is kinda broken. I'm not saying I know how you could have done any better, but I do think we're going to have to try to figure out something to do about it, because saying, "check-pg_upgrade is 4x slower, but that's just because of all those bad estimates" is not going to fly. Those bad estimates were harmlessly bad before, and now they are harmfully bad, and similar bad estimates are going to exist in real-world queries, and those are going to be harmful now too. Blaming the bad costing is a red herring. The problem is that you've made the costing matter in a way that it previously didn't. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
