On Tue, Jan 10, 2017 at 10:55 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Wed, Dec 28, 2016 at 1:17 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: >> Currently, queries that have references to SubPlans or >> AlternativeSubPlans are considered parallel-restricted. I think we >> can lift this restriction in many cases especially when SubPlans are >> parallel-safe. To make this work, we need to propagate the >> parallel-safety information from path node to plan node and the same >> could be easily done while creating a plan. Another option could be >> that instead of propagating parallel-safety information from path to >> plan, we can find out from the plan if it is parallel-safe (doesn't >> contain any parallel-aware node) by traversing whole plan tree, but I >> think it is a waste of cycles. Once we have parallel-safety >> information in the plan, we can use that for detection of >> parallel-safe expressions in max_parallel_hazard_walker(). Finally, >> we can pass all the subplans to workers during plan serialization in >> ExecSerializePlan(). This will enable workers to execute subplans >> that are referred in parallel part of the plan. Now, we might be able >> to optimize it such that we pass only subplans that are referred in >> parallel portion of plan, but I am not sure if it is worth the trouble >> because it is one-time cost and much lesser than other things we do >> (like creating >> dsm, launching workers). > > It seems unfortunate to have to add a parallel_safe flag to the > finished plan; the whole reason we have the Path-Plan distinction is > so that we can throw away information that won't be needed at > execution time. The parallel_safe flag is, in fact, not needed at > execution time, but just for further planning. Isn't there some way > that we can remember, at the time when a sublink is converted to a > subplan, whether or not the subplan was created from a parallel-safe > path? >
The other alternative is to remember this information in SubPlan. We can retrieve parallel_safe information from best_path and can use it while generating SubPlan. The main reason for storing it in the plan was to avoid explicitly passing parallel_safe information while generating SubPlan as plan was already available at that time. However, it seems there are only two places in code (refer build_subplan) where this information needs to be propagated. Let me know if you prefer to remember the parallel_safe information in SubPlan instead of in Plan or if you have something else in mind? -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
