On Tue, Jan 4, 2022 at 5:31 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > > Greg Nancarrow <gregn4...@gmail.com> writes: > > The patch LGTM. > > I have set the status to "Ready for Committer". > > I don't really see why this patch is even a little bit safe. > The argument for it seems to be that a lateral subquery will > necessarily be executed in such a way that each complete iteration > of the subquery, plus joining to its outer rel, happens within a > single worker ... but where is the guarantee of that? Once > you've marked the rel as parallel-safe, the planner is free to > consider all sorts of parallel join structures. I'm afraid this > would be easily broken as soon as you look at cases with three or > more rels. Or maybe even just two. The reason for the existing > restriction boils down to this structure being unsafe: > > Gather > NestLoop > Scan ... > Limit > Scan ... > > and I don't see how the existence of a lateral reference > makes it any safer.
Thanks for taking a look. I'm not following how the structure you posited is inherently unsafe when it's a lateral reference. That limit/scan (if lateral) has to be being executed per tuple in the outer scan, right? And if it's a unique execution per tuple, then consistency across tuples (that are in different workers) can't be a concern. Is there a scenario I'm missing where lateral can currently be executed in that way in that structure (or a different one)? Thanks, James Coleman
