On Sun, Jun 26, 2016 at 10:35 PM, Noah Misch <n...@leadboat.com> wrote: >> I looked into this and found that the costs are considered fuzzily the >> same, and then add_path prefers the slightly-worse path on the grounds >> that it is marked parallel_safe while the MinMaxAgg path is not. It seems >> to me that there is some fuzzy thinking going on there. On exactly what >> grounds is a path to be preferred merely because it is parallel safe, and >> not actually parallelized? Or perhaps the question to ask is whether a >> MinMaxAgg path can be marked parallel-safe. > > [Action required within 72 hours. This is a generic notification.] > > The above-described topic is currently a PostgreSQL 9.6 open item ("consider > whether MinMaxAggPath might fail to be parallel-safe"). Robert, since you > committed the patch believed to have created it, you own this open item. If > some other commit is more relevant or if this does not belong as a 9.6 open > item, please let us know. Otherwise, please observe the policy on open item > ownership[1] and send a status update within 72 hours of this message. > Include a date for your subsequent status update. Testers may discover new > open items at any time, and I want to plan to get them all fixed well in > advance of shipping 9.6rc1. Consequently, I will appreciate your efforts > toward speedy resolution. Thanks.
It turns out that this open item is phased incorrectly. I'll update the phrasing. /* A MinMaxAggPath implies use of subplans, so cannot be parallel-safe */ pathnode->path.parallel_safe = false; Currently, MinMaxAggPath is never parallel-safe; the question is whether we could allow it to be parallel-safe (not, as the current phrasing implies, whether it might ever need to be other than parallel-safe). It appears to me that the answer is "no", because a MinMaxAggPath contains a list of MinMaxAggInfo objects, and there we have this: Param *param; /* param for subplan's output */ Since subplans aren't passed down to parallel workers, no MinMaxAggPath can be parallel-safe. Therefore, I think there's nothing to do here right now. Comments? See also https://www.postgresql.org/message-id/ca+tgmoz7wvmpmtsntk+dfdunwmc8kk5putldnzolvj9dvea...@mail.gmail.com (Official status update: I'll remove this open item in 3 days unless the above analysis is shown to be incorrect.) -- 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