Why is DEFAULT_FDW_TUPLE_COST so insanely low?
- Jump to firstname.lastname@example.orgT14:56:12+00:00Over on  I was complaining that I thought DEFAULT_FDW_TUPLE_COST, which is defined as 0.01 was unrealistically low. For comparison, cpu_tuple_cost, something we probably expect to be in a CPU cache is also 0.01. We've defined DEFAULT_PARALLEL_TUPLE_COST to be 0.1, which is 10x cpu_tuple_cost. That's coming from a shared memory segment. So why do we think DEFAULT_FDW_TUPLE_COST should be the same as cpu_tuple_cost when that's probably pulling a tuple from some remote server over some (possibly slow) network? I did a little experiment in the attached .sql file and did some maths to try to figure out what it's really likely to be costing us. I tried this with and without the attached hack to have the planner not consider remote grouping just to see how much slower pulling a million tuples through the FDW would cost. I setup a loopback server on localhost (which has about the lowest possible network latency) and found the patched query to the foreign server took: Execution Time: 530.000 ms This is pulling all million tuples over and doing the aggregate locally. Unpatched, the query took: Execution Time: 35.334 ms so about 15x faster. If I take the seqscan cost for querying the local table, which is 14425.00 multiply that by 15 (the extra time it took to pull the 1 million tuples) then divide by 1 million to get the extra cost per tuple, then that comes to about 0.216. So that says DEFAULT_FDW_TUPLE_COST is about 21x lower than it should be. I tried cranking DEFAULT_FDW_TUPLE_COST up to 0.5 to see what plans would change in the postgres_fdw regression tests and quite a number changed. Many seem to be pushing the sorts down to the remote server where they were being done locally before. A few others just seem weird. For example, the first one seems to be blindly adding a remote sort when it does no good. I think it would take quite a bit of study with a debugger to figure out what's going on with many of these. Does anyone have any ideas why DEFAULT_FDW_TUPLE_COST was set so low? Does anyone object to it being set to something more realistic? David  https://www.postgresql.org/message-id/CAApHDvpXiXLxg4TsA8P_4etnuGQqAAbHWEOM4hGe=DCaXmi_jA@mail.gmail.com