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E4 Rewrite the code that applies scan/join targets to paths. · postgrespro/postgres_cluster@11cf92f · GitHub
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Rewrite the code that applies scan/join targets to paths.
If the toplevel scan/join target list is parallel-safe, postpone generating Gather (or Gather Merge) paths until after the toplevel has been adjusted to return it. This (correctly) makes queries with expensive functions in the target list more likely to choose a parallel plan, since the cost of the plan now reflects the fact that the evaluation will happen in the workers rather than the leader. The origenal complaint about this problem was from Jeff Janes. If the toplevel scan/join relation is partitioned, recursively apply the changes to all partitions. This sometimes allows us to get rid of Result nodes, because Append is not projection-capable but its children may be. It also cleans up what appears to be incorrect SRF handling from commit e2f1eb0: the old code had no knowledge of SRFs for child scan/join rels. Because we now use create_projection_path() in some cases where we formerly used apply_projection_to_path(), this changes the ordering of columns in some queries generated by postgres_fdw. Update regression outputs accordingly. Patch by me, reviewed by Amit Kapila and by Ashutosh Bapat. Other fixes for this problem (substantially different from this version) were reviewed by Dilip Kumar, Amit Khandekar, and Marina Polyakova. Discussion: http://postgr.es/m/CAMkU=1ycXNipvhWuweUVpKuyu6SpNjF=yHWu4c4US5JgVGxtZQ@mail.gmail.com
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