Greetings, * Tom Lane (tgl@xxxxxxxxxxxxx) wrote: > shigeo Hirose <shigeo.hirose@xxxxxxxxxxxxx> writes: > > How can I pushdown of functions used in targetlist with FDW ? > > There is, AFAIK, no provision for that. There's not a lot of > reason to consider adding it either, because there's no reason > to suppose that the remote node can run such a function any > faster than the local node. So the difficulty and risk of > determining/assuming that f(x) on the local node is the same > as f(x) on the remote node doesn't seem like it'd be repaid. Well, for starters, isn't there something in the SQL/MED spec about this..? Next, we already do this for aggregates. Also, I don't think I agree with this particular position- it's entirely possible that the remote server is much less loaded/busy than the local one and therefore it'd be beneficial, overall, to run that function on the remote system and not the local one. The function might even have side-effects or use other objects in the system to run. The function may also return a much smaller result than pulling back the raw data (of course, the opposite could also possibly be true). I guess my gut feeling is that, in general, we should push down as much of the query as possible, as we do for aggregates and joins and conditionals. I appreciate that there's some complications here when it comes to figuring out if it's possible/reasonable to push down something like, say, a volatile function that's locally defined and known to exist on the remote. Haven't got a particular idea how to address that offhand but that doesn't change my feelings that we should have a way to do this generally and that it would actually be useful to have. Thanks, Stephen
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