On Fri, Jan 13, 2017 at 3:22 AM, Etsuro Fujita <fujita.ets...@lab.ntt.co.jp> wrote:
> On 2017/01/13 0:43, Robert Haas wrote: > >> On Wed, Jan 11, 2017 at 2:45 AM, Etsuro Fujita >> <fujita.ets...@lab.ntt.co.jp> wrote: >> >>> As I said before, that might be fine for 9.6, but I don't think it's a >>> good >>> idea to search the pathlist because once we support parameterized foreign >>> join paths, which is on my TODOs, we would have to traverse through the >>> possibly-lengthy pathlist to find a local-join path, as mentioned in [3]. >>> >> > I'm not sure that's really going to be a problem. The number of >> possible parameterizations that need to be considered isn't usually >> very big. I bet the path list will have ten or a few tens of entries >> in it, not a hundred or a thousand. Traversing it isn't that >> expensive. >> >> That having been said, I haven't read the patches, so I'm not really >> up to speed on the bigger issues here. But surely it's more important >> to get the overall design right than to worry about the cost of >> walking the pathlist or worrying about the cost of an extra function >> call (?!). >> > > My biggest concern about GetExistingLocalJoinPath is that might not be > extendable to the case of foreign-join paths with parameterization; in > which case, fdw_outerpath for a given foreign-join path would need to have > the same parameterization as the foreign-join path, but there might not be > any existing paths with the same parameterization in the path list. You > might think we could get the fdw_outerpath by getting an existing path with > no parameterization as in GetExistingLocalJoinPath and then modifying the > path's param_info to match the parameterization of the foreign-join path. > I don't know that really works, but that might be inefficient. > > What I have in mind to support foreign-join paths with parameterization > for postgres_fdw like this: (1) generate parameterized paths from any > joinable combination of the outer/inner cheapest-parameterized paths that > have pushed down the outer/inner relation to the remote server, in a > similar way as postgresGetForeignJoinPaths creates unparameterized paths, > and (2) create fdw_outerpath for each parameterized path from the > outer/inner paths used to generate the parameterized path, by > create_nestloop_path (or, create_hashjoin_path or create_mergejoin_path if > full join), so that the resulting fdw_outerpath has the same > parameterization as the paramterized path. This would probably work and > might be more efficient. And the patch I proposed would be easily extended > to this, by replacing the outer/inner cheapest-total paths with the > outer/inner cheapest-parameterized paths. Attached is the latest version > of the patch. > I'm afraid this discussion and the C code here are over my head. But I've tested this patch against 9.6.stable-2d443ae1b0121e15265864d2b21 and 10.dev-0563a3a8b59150bf3cc8, and in both cases it fixes the original problem. I do get a compiler warning: foreign.c: In function 'CreateLocalJoinPath': foreign.c:832: warning: implicit declaration of function 'pathkeys_contained_in' Cheers, Jeff
