On Tue, Mar 29, 2022 at 10:06 PM David Rowley <dgrowle...@gmail.com> wrote:
> On Wed, 30 Mar 2022 at 02:38, Robert Haas <robertmh...@gmail.com> wrote: > > I think WARNING is fine. After all, the parameter is called > > "jit_warn_above_fraction". > > I had a think about this patch. I guess it's a little similar to > checkpoint_warning. The good thing about the checkpoint_warning is > that in the LOG message we give instructions about how the DBA can fix > the issue, i.e increase max_wal_size. > > With the proposed patch I see there is no hint about what might be > done to remove/reduce the warnings. I imagine that's because it's not > all that clear which GUC should be changed. In my view, likely > jit_above_cost is the most relevant but there is also > jit_inline_above_cost, jit_optimize_above_cost, jit_tuple_deforming > and jit_expressions which are relevant too. > > If we go with this patch, the problem I see here is that the amount > of work the JIT compiler must do for a given query depends mostly on > the number of expressions that must be compiled in the query (also to > a lesser extent jit_inline_above_cost, jit_optimize_above_cost, > jit_tuple_deforming and jit_expressions). The DBA does not really have > much control over the number of expressions in the query. All he or > she can do to get rid of the warning is something like increase > jit_above_cost. After a few iterations of that, the end result is > that jit_above_cost is now high enough that JIT no longer triggers > for, say, that query to that table with 1000 partitions where no > plan-time pruning takes place. Is that really a good thing? It likely > means that we just rarely JIT anything at all! > I don't agree with the conclusion of that. What the parameter would be useful for is to be able to tune those costs (or just turn it off) *for that individual query*. That doesn't mean you "rarely JIT anything atll", it just means you rarely JIT that particular query. In fact, my goal is to specifically make people do that and *not* just turn off JIT globally. I'd much rather see us address the costing problem before adding some > warning, especially a warning where it's not clear how to make go > away. > The easiest way would be to add a HINT that says turn off jit for this particular query or something? I do agree that if we can make "spending too much time on JIT vs query runtime" go away completely, then there is no need for a parameter like this. I still think the warning is useful. And I think it may stay useful even after we have made the JIT costing smarter -- though that's not certain of course. -- Magnus Hagander Me: https://www.hagander.net/ <http://www.hagander.net/> Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>
