On Fri, Jul 16, 2021 at 3:16 PM James Coleman <jtc...@gmail.com> wrote: > > On Thu, May 27, 2021 at 9:01 PM Greg Nancarrow <gregn4...@gmail.com> wrote: > > > > On Tue, Dec 8, 2020 at 10:46 AM James Coleman <jtc...@gmail.com> wrote: > > > > > > While I haven't actually tracked down to guarantee this is handled > > > elsewhere, a thought experiment -- I think -- shows it must be so. > > > Here's why: suppose we don't have a limit here, but the query return > > > order is different in different backends. Then we would have the same > > > problem you bring up. In that case this code is already setting > > > consider_parallel=true on the rel. So I don't think we're changing any > > > behavior here. > > > > > > > AFAICS, the patch seems very reasonable and specifically targets > > lateral subqueries with limit/offset. It seems like the uncorrelated > > case is the only real concern. > > I generally agree that the current patch is probably not changing any > > behavior in the uncorrelated case (and like yourself, haven't yet > > found a case for which it breaks), but I'm not sure Brian's concerns > > can be ruled out entirely. > > > > How about a minor update to the patch to make it slightly more > > restrictive, to exclude the case when there are no lateral > > cross-references, so we'll be allowing parallelism only when we know > > the lateral subquery will be evaluated anew for each source row? > > I was thinking of the following patch modification: > > > > BEFORE: > > - if (limit_needed(subquery)) > > + if (!rte->lateral && limit_needed(subquery)) > > > > AFTER: > > - if (limit_needed(subquery)) > > + if ((!rte->lateral || bms_is_empty(rel->lateral_relids)) && > > + limit_needed(subquery)) > > > > > > Thoughts? > > Apologies for the delayed response; this seems fine to me. I've > attached patch v2.
Greg, Do you believe this is now ready for committer? Thanks, James Coleman
