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? Regards, Greg Nancarrow Fujitsu Australia
