On Fri, Nov 24, 2023 at 3:59 AM David Rowley <dgrowle...@gmail.com> wrote: > > Another problem I hit was add_path() pfreeing a Path that I needed. > This was happening due to how I'm building the final paths in the > subquery when setop_pathkeys are set. Because I want to always > include the cheapest_input_path to allow that path to be used in > hash-based UNIONs, I also want to provide sorted paths so that > MergeAppend has something to work with. I found cases where I'd > add_path() the cheapest_input_path to the final rel then also attempt > to sort that path. Unfortunately, add_path() found the unsorted path > and the sorted path fuzzily the same cost and opted to keep the sorted > one due to it having better pathkeys. add_path() then pfree'd the > cheapest_input_path which meant the Sort's subpath was gone which > obviously caused issues in createplan.c. > > For now, as a temporary fix, I've just #ifdef'd out the code in > add_path() that's pfreeing the old path. I have drafted a patch that > refcounts Paths, but I'm unsure if that's the correct solution as I'm > only maintaining the refcounts in add_path() and add_partial_path(). I > think a true correct solution would bump the refcount when a path is > used as some other path's subpath. That would mean having to > recursively pfree paths up until we find one with a refcount>0. Seems > a bit expensive for add_path() to do.
Please find my proposal to refcount paths at [1]. I did that to reduce the memory consumed by partitionwise joins. I remember another thread where freeing a path that was referenced by upper sort path created minor debugging problem. [2]. I paused my work on my proposal since there didn't seem enough justification. But it looks like the requirement is coming up repeatedly. I am willing to resume my work if it's needed. The email lists next TODOs. As to making the add_path() expensive, I didn't find any noticeable impact on planning time. [1] https://www.postgresql.org/message-id/CAExHW5tUcVsBkq9qT%3DL5vYz4e-cwQNw%3DKAGJrtSyzOp3F%3DXacA%40mail.gmail.com [2] https://www.postgresql.org/message-id/CAM2%2B6%3DUC1mcVtM0Y_LEMBEGHTM58HEkqHPn7vau_V_YfuZjEGg%40mail.gmail.com -- Best Wishes, Ashutosh Bapat
