On Tue, Mar 29, 2022 at 1:06 PM David Rowley <dgrowle...@gmail.com> wrote: > That means that even if the total execution time of a plan was a true > reflection of the total estimated plan cost, then the fraction of time > spent (as is measured by jit_warn_above_fraction) doing JIT would > entirely depend on the number of expressions to compile. Of course, > the planner's not that good, but does that not indicate that the JIT > costing should really account for the number of expressions and not > just the total plan cost?
That's a good point. The difference between the actual cost of executing the query with and without JIT'ing is what we care about, for the most part. Maybe we could do a lot better just by inventing a fairly crude model that captures the benefits of JIT'ing -- that's what "counting the number of expressions" sounds like to me. This model could probably assume that JIT'ing itself was free -- maybe something this simple would work well. The planner has traditionally used the cost units to determine the cheapest plan; it compared total plan cost for plans that were taken from the universe of possible plans for *one specific query*. That's completely different to any model that expects plan costs to be meaningful in an absolute sense. I'm not completely sure how much that difference matters, but I suspect that the answer is: "it depends, but often it matters a great deal". -- Peter Geoghegan
