> On Fri, Mar 13, 2015 at 2:31 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Robert Haas <robertmh...@gmail.com> writes: > >> Another bit of this that I think we could commit without fretting > >> about it too much is the code adding set_join_pathlist_hook. This is > >> - I think - analogous to set_rel_pathlist_hook, and like that hook, > >> could be used for other purposes than custom plan generation - e.g. to > >> delete paths we do not want to use. I've extracted this portion of > >> the patch and adjusted the comments; if there are no objections, I > >> will commit this bit also. > > > > I don't object to the concept, but I think that is a pretty bad place > > to put the hook call: add_paths_to_joinrel is typically called multiple > > (perhaps *many*) times per joinrel and thus this placement would force > > any user of the hook to do a lot of repetitive work. > > Interesting point. I guess the question is whether a some or all > callers are going to actually *want* a separate call for each > invocation of add_paths_to_joinrel(), or whether they'll be happy to > operate on the otherwise-complete path list. It's true that if your > goal is to delete paths, it's probably best to be called just once > after the path list is complete, and there might be a use case for > that, but I guess it's less useful than for baserels. For a baserel, > as long as you don't nuke the sequential-scan path, there is always > going to be a way to complete the plan; so this would be a fine way to > implement a disable-an-index extension. But for joinrels, it's not so > easy to rule out, say, a hash-join here. Neither hook placement is > much good for that; the path you want to get rid of may have already > dominated paths you want to keep. > From the standpoint of extension development, I'm uncertain whether we can easily reproduce information needed to compute alternative paths on the hook at standard_join_search(), like a hook at add_paths_to_joinrel().
(Please correct me, if I misunderstood.) For example, it is not obvious which path is inner/outer of the joinrel on which custom-scan provider tries to add an alternative scan path. Probably, extension needs to find out the path of source relations from the join_rel_level[] array. Also, how do we pull SpecialJoinInfo? It contains needed information to identify required join-type (like JOIN_LEFT), however, extension needs to search join_info_list by relids again, if hook is located at standard_join_search(). Even if number of hook invocation is larger if it is located on add_paths_to_joinrel(), it allows to design extensions simpler, I think. > Suppose you want to add paths - e.g. you have an extension that goes > and looks for a materialized view that matches this subtree of the > query, and if it finds one, it substitutes a scan of the materialized > view for a scan of the baserel. Or, as in KaiGai's case, you have an > extension that can perform the whole join in GPU-land and produce the > same results we would have gotten via normal execution. Either way, > you want - and this is the central point of the whole patch here - to > inject a scan path into a joinrel. It is not altogether obvious to me > what the best placement for this is. In the materialized view case, > you probably need a perfect match between the baserels in the view and > the baserels in the joinrel to do anything. There's no point in > re-checking that for every innerrels/outerrels combination. I don't > know enough about the GPU case to reason about it intelligently; maybe > KaiGai can comment. > In case of GPU, extension will add alternative paths based on hash-join and nested-loop algorithm with individual cost estimation as long as device can execute join condition. It expects planner (set_cheapest) will choose the best path in the built-in/additional ones. So, it is more reasonable for me, if extension can utilize a common infrastructure as built-in logic (hash-join/merge-join/nested-loop) is using to compute its cost estimation. > But there's another possible approach: suppose that > join_search_one_level, after considering left-sided and right-sided > joins and after considering bushy joins, checks whether every relation > it's got is from the same foreign server, and if so, asks that foreign > server whether it would like to contribute any paths. Would that be > better or worse? A disadvantage is that if you've got something like > A LEFT JOIN B LEFT JOIN C LEFT JOIN D LEFT JOIN E LEFT JOIN F LEFT > JOIN G LEFT JOIN H LEFT JOIN I but none of the joins can be pushed > down (say, each join clause calls a non-pushdown-safe function) you'll > end up examining a pile of joinrels - at every level of the join tree > - and individually rejecting each one. With the > build-it-up-incrementally approach, you'll figure that all out at > level 2, and then after that there's nothing to do but give up > quickly. On the other hand, I'm afraid the incremental approach might > miss a trick: consider small LEFT JOIN (big INNER JOIN huge ON big.x = > huge.x) ON small.y = big.y AND small.z = huge.z, where all three are > foreign tables on the same server. If the output of the big/huge join > is big, none of those paths are going to survive at level 2, but the > overall join size might be very small, so we surely want a chance to > recover at level 3. (We discussed test cases of this form quite a bit > in the context of e2fa76d80ba571d4de8992de6386536867250474.) > > Thoughts? > Do we need to pay attention on relids of joinrel, instead of innerpath and outerpath? Yep, we might assume a path with join pushed-down has cheaper cost than combination of two foreign-scan and a local join, however, foreign-scan with join pushed-down may partially have expensive cost. In this case, either of hook location may be reasonable, because FDW driver can check whether all the relids are foreign-scan path managed by same foreign-server, or not, regardless of innerpath/outerpath. Of course, it is a significant factor for extensions (including FDW driver) whether hook allows to utilize a common infrastructure (like SpecialJoinInfo or join restrictlist, ...). Thanks, -- NEC OSS Promotion Center / PG-Strom Project KaiGai Kohei <kai...@ak.jp.nec.com> -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
