On Thu, Dec 30, 2021 at 4:44 PM Yura Sokolov <y.soko...@postgrespro.ru> wrote:
> Good day, hackers. > > Problem: > - Append path is created with explicitely parallel_aware = true > - It has two child, one is trivial, other is parallel_aware = false . > Trivial child is dropped. > - Gather/GatherMerge path takes Append path as a child and thinks > its child is parallel_aware = true. > - But Append path is removed at the last since it has only one child. > - Now Gather/GatherMerge thinks its child is parallel_aware, but it > is not. > Gather/GatherMerge runs its child twice: in a worker and in a leader, > and gathers same rows twice. > > Reproduction code attached (repro.sql. Included as a test as well). > Yeah, this is a problem. > > Suggested quick (and valid) fix in the patch attached: > - If Append has single child, then copy its parallel awareness. > > Bug were introduced with commit 8edd0e79460b414b1d971895312e549e95e12e4f > "Suppress Append and MergeAppend plan nodes that have a single child." > > During discussion, it were supposed [1] those fields should be copied: > > > I haven't looked into whether this does the right things for parallel > > planning --- possibly create_[merge]append_path need to propagate up > > parallel-related path fields from the single child? > > But it were not so obvious [2]. > > Better fix could contain removing Gather/GatherMerge node as well if > its child is not parallel aware. > The Gather path will only be created if we have an underlying partial path, so I think if we are generating the append path only from the non-partial paths then we can see if the number of child nodes is just 1 then don't generate the partial append path, so from that you will node generate the partial join and eventually gather will be avoided. -- Regards, Dilip Kumar EnterpriseDB: http://www.enterprisedb.com
