I believe that we should be delaying the PlannerInfo's total_table_pages calculation until after constraint exclusion and partition pruning have taken place. Doing this calculation before we determine which relations we don't need to scan can lead to incorrectly applying random_page_cost to too many pages processed during an Index Scan.
We already don't count relations removed by join removals from this calculation, so counting pruned partitions seems like an omission. The attached patch moves the calculation to after set_base_rel_sizes() is called and before set_base_rel_pathlists() is called, where the information is actually used. I am considering this a bug fix, but I'm proposing this for PG12 only as I don't think destabilising plans in the back branches is a good idea. I'll add this to the September commitfest. -- David Rowley http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
v1-0001-Calculate-total_table_pages-after-set_base_rel_si.patch
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