On Mon, Dec 29, 2025 at 5:15 PM Lukas Fittl <[email protected]> wrote:
> On Mon, Dec 15, 2025 at 12:06 PM Robert Haas <[email protected]> > wrote: > > Here's v7. > > I'm excited about this patch series, and in an effort to help land the > infrastructure, here is a review of 0001 - 0003 to start: > For the review of 0004, I decided to spend a few days to test if the plan generation strategy logic will work for pg_hint_plan, as an existing extension in the ecosystem that is widely used, but functions today by virtue of modifying planner GUCs whilst the planner is running. First of all, as is expected, the extension completely stops working with 0004 in place - because we now only read the GUCs at planner start, the mechanism of modifying them in the middle doesn't work. I don't think we can avoid this. That said, good news: After a bunch of iterations, I get a clean pass on the pg_hint_plan regression tests, whilst completely dropping its copying of core code and hackish re-run of set_plain_rel_pathlist. See [0] for a draft PR (on my own fork of pg_hint_plan) with individual patches that explain some regression test differences. Adding Michael in CC, since he's been thankfully maintaining pg_hint_plan over the years, and I think if 0004 gets merged that should significantly reduce the maintenance burden, independently of what happens with pg_plan_advice - so his input would be useful here. The biggest change in the regression test output was due to how the "Parallel" hint worked in pg_hint_plan (basically it was setting parallel_*_cost to zero, and then messed with the gucs that factor into compute_parallel_worker) -- I think the only sensible thing to do is to change that in pg_hint_plan, and instead rely on rejecting non-partial paths with PGS_CONSIDER_NONPARTIAL if "hard" enforcement of parallelism is requested. That caused some minor plan changes, but I think they can still be argued to be matching the user's intent of "make a scan involving this relation parallel". There were two bugs in 0004 that I had to fix to make this work: In cost_index, we are checking "path->path.parallel_workers == 0", but parallel_workers only gets set later in the function, causing the PGS_CONSIDER_NONPARTIAL mask to not be applied. Replacing this with checking the "partial_path" argument instead makes it work. In cost_samplescan, we set the PGS_CONSIDER_NONPARTIAL mask if its a non-partial path, but that causes Sample Scans to always be disabled when setting PGS_CONSIDER_NONPARTIAL on the relation. I think we could simply drop that check, since we never generate partial sample scan paths. Otherwise 0004 looks good to me, and the mechanism of working with mask values felt natural to me, especially in contrast with existing ways to achieve something similar. I did not test partition wise joins, since pg_hint_plan doesn't cover them today. [0]: https://github.com/lfittl/pg_hint_plan/pull/1 Thanks, Lukas -- Lukas Fittl
