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? That seems like it would be cleaner than maintaining this flag for all plans. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
