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The previous ordering of these steps satisfied the nominal requirement that set_rel_pathlist_hook could editorialize on the whole set of Paths constructed for a base relation. In practice, though, trying to change the set of partial paths was impossible. Adding one didn't work because (a) it was too late to be included in Gather paths made by the core code, and (b) calling add_partial_path after generate_gather_paths is unsafe, because it might try to delete a path it thinks is dominated, but that is already embedded in some Gather path(s). Nor could the hook safely remove partial paths, for the same reason that they might already be embedded in Gathers. Better to call extensions first, let them add partial paths as desired, and then gather. In v11 and up, we already doubled down on that ordering by postponing gathering even further for single-relation queries; so even if the hook wished to editorialize on Gather path construction, it could not. Report and patch by KaiGai Kohei. Back-patch to 9.6 where Gather paths were added. Discussion: https://postgr.es/m/CAOP8fzahwpKJRTVVTqo2AE=mDTz_efVzV6Get_0=U3SO+-ha1A@mail.gmail.com |
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