On Mon, Jan 29, 2024 at 6:11 PM Alexander Kuzmenkov <akuzmen...@timescale.com> wrote: > > Hello hackers, > > While investigating some query plans, I noticed some code that seems > to be wrong: when create_merge_append_path() estimates the cost of > sorting an input, it calls cost_sort() passing subpath->parent->tuples > as the number of tuples. Shouldn't it use subpath->parent->rows or > even subpath->rows instead? The `tuples` variable doesn't account for > the filters on the relation, so this leads to incorrect cost estimates > when there are highly selective filters, and Sort + Append is chosen > instead of MergeAppend.
All other callers of cost_sort() except plan_cluster_use_sort() are using rows instead of tuples. Even plan_cluster_use_sort() has rel->rows = rel->tuples, it's actually passing rows. So agree with your suggestion. However a test will be good since this code is quite old. -- Best Wishes, Ashutosh Bapat
