Hi, On November 30, 2022 3:47:32 AM PST, Andrew Dunstan <andrew@xxxxxxxxxxxx> wrote: > >On 2022-11-29 Tu 16:06, David Rowley wrote: >> On Wed, 30 Nov 2022 at 03:31, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: >>> Alvaro Herrera <alvherre@xxxxxxxxxxxxxx> writes: >>>> IMO it was a mistake to turn JIT on in the default config, so that's one >>>> thing you'll likely want to change. >>> I wouldn't necessarily go quite that far, but I do think that the >>> default cost thresholds for invoking it are enormously too low, >>> or else there are serious bugs in the cost-estimation algorithms >>> for deciding when to use it. A nearby example[1] of a sub-1-sec >>> partitioned query that took 30sec after JIT was enabled makes me >>> wonder if we're accounting correctly for per-partition JIT costs. >> I'm very grateful for JIT. However, I do agree that the costs need to work. >> >> The problem is that the threshold to turn JIT on does not consider how >> many expressions need to be compiled. It's quite different to JIT >> compile a simple one-node plan with a total cost of 100000 than to JIT >> compile a plan that costs the same but queries 1000 partitions. I >> think we should be compiling expressions based on the cost of the >> individial node rather than the total cost of the plan. We need to >> make some changes so we can more easily determine the number of times >> a given node will be executed before we can determine how worthwhile >> JITting an expression in a node will be. >> > >I think Alvaro's point is that it would have been better to work out >these wrinkles before turning on JIT by default. Based on anecdotal >reports from the field I'm inclined to agree. The problem is that back when it was introduced these problems didn't exist to a significant degree. JIT was developed when partitioning was very minimal- and the problems we're seeing are almost exclusively with queries with many partitions. The problems really only started much more recently. It also wasn't enabled in the first release.. Andres -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
