On 2019/03/28 14:03, Tom Lane wrote: > Amit Langote <langote_amit...@lab.ntt.co.jp> writes: >> On 2019/03/27 23:57, Tom Lane wrote: >>> Yeah, there's something to be said for having plancat.c open each table >>> *and store the Relation pointer in the RelOptInfo*, and then close that >>> again at the end of planning rather than immediately. If we can't avoid >>> these retail table_opens without a great deal of pain, that's the >>> direction I'd tend to go in. However it would add some overhead, in >>> the form of a need to traverse the RelOptInfo array an additional time. > >> Just to be sure, do you mean we should do that now or later (David said >> "in the long term")? > > It's probably not high priority, though I wonder how much time is being > eaten by the repeated table_opens.
It took me a couple of hours to confirm this, but it seems significantly less than it takes to go over the simple_rel_array one more time to close the relations. The scan of simple_rel_array to close the relations needs to be done once per query_planner() invocation, whereas I'd hoped this could be done, say, in add_rtes_to_flat_rtable() that has to iterate over the range table anyway. That's because we call query_planner multiple times on the same query in a few cases, viz. build_minmax_path(), inheritance_planner(). Within query_planner(), table is opened in expand_inherited_rtentry(), get_relation_constraints(), get_relation_data_width(), and build_physical_tlist(), of which the first two are more common. So, we end up doing table_open() 3 times on average for any given relation, just inside query_planner(). Thanks, Amit
