On 12/4/10 2:40 PM, Tom Lane wrote: > [ pokes at that for a bit ... ] Ah, I had forgotten that UPDATE/DELETE > go through inheritance_planner() while SELECT doesn't. And > inheritance_planner() makes a copy of the querytree, including the > already-expanded range table, for each target relation. So the memory > usage is O(N^2) in the number of child tables.
Thanks for the pointer to the code and explanation. In inheritance_planner(...) I see the memcpy of the input query tree, but for my example constraint exclusion would only result in one child being included. Or is the O(N^2) memory usage from elsewhere? > It's difficult to do much better than that in the general case where the > children might have different rowtypes from the parent: you need a > distinct targetlist for each target relation. I expect that we can be a > lot smarter when we have true partitioning support (which among other > things is going to have to enforce that all the children have identical > column sets). Is this the same as saying that the inheritance_planner(...) can be avoided if it were known that the children have the same rowtype as the parent? Is it easy to check? > But the inheritance mechanism was never intended to scale to anything like > this number of children. Unfortunately so. :( When I push the number of child tables up to 10k, the SELECT planning starts to slow down (~1 sec), though no swapping. > I remain of the opinion that you're using far too many child tables. > Please note the statement at the bottom of > http://www.postgresql.org/docs/9.0/static/ddl-partitioning.html: > > Partitioning using these techniques will work well with up to > perhaps a hundred partitions; don't try to use many thousands of > partitions. Thanks for the reference---I'm well aware of it, but it was not clear to me why: the reason I was structuring my partition inheritance as a tree, because I thought it was simply a case of time-to-scan the CHECK constraints at any level in the inheritance hierarchy. You've been a great help in helping my understanding PostgreSQL inheritance. Best, John -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
