On 11 April 2018 at 09:32, Alvaro Herrera <alvhe...@alvh.no-ip.org> wrote: > Robert Haas wrote: >> On Mon, Apr 9, 2018 at 2:28 PM, Alvaro Herrera <alvhe...@alvh.no-ip.org> >> wrote: > >> >> I don't get this. The executor surely had to (and did) open all of >> >> the relations somewhere even before this patch. > >> > I was worried that this coding could be seen as breaking modularity, or >> > trying to do excessive work. However, after looking closer at it, it >> > doesn't really look like it's the case. So, nevermind. >> >> Well what I'm saying is that it shouldn't be necessary. If the >> relations are being opened already and the pointers to the relcache >> entries are being saved someplace, you shouldn't need to re-open them >> elsewhere to get pointers to the relcache entries. > > I looked a bit more into this. It turns out that we have indeed opened > the relation before -- first in parserOpenTable (for addRangeTableEntry), > then in expandRTE, then in QueryRewrite, then in subquery_planner, then > in get_relation_info. > > So, frankly, since each module thinks it's okay to open it every once in > a while, I'm not sure we should be terribly stressed about doing it once > more for partition pruning. Particularly since communicating the > pointer seems to be quite troublesome.
I guess the problem there would be there's nothing to say that parse analysis will shortly be followed by a call to the planner, and a call to the planner does not mean the plan is about to be executed. So I don't think it would be possible to keep pointers to relcache entries between these modules, and it would be hard to determine whose responsibility it would be to call relation_close(). It might be possible to do something better in each module by keeping an array indexed by RTI which have each entry NULL initially then on first relation_open set the element in the array to that pointer. This might mean we'd save a few relation_open calls, but I don't know if there would be a way to somehow remove the Relation from the array on relation_close. Having something like this might mean we could detect lock upgrade hazards more easily, but the whole thing is a cache on top of a cache which does seem a bit weird. relation_open() should be pretty cheap if the relation is already open. It's just a hash table lookup. What is described above just changes that to an array lookup. It also does nothing for index_open. However, something like the above would simplify ExecLockNonLeafAppendTables() a bit and get rid of the O(N^2) which checks the partition is not a result relation. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services