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In the initial revision of the upper-planner pathification work, the only available way for an FDW or custom-scan provider to inject Paths representing post-scan-join processing was to insert them during scan-level GetForeignPaths or similar processing. While that's not impossible, it'd require quite a lot of duplicative processing to look forward and see if the extension would be capable of implementing the whole query. To improve matters for custom-scan providers, provide a hook function at the point where the core code is about to start filling in upperrel Paths. At this point Paths are available for the whole scan/join tree, which should reduce the amount of redundant effort considerably. (An alternative design that was suggested was to provide a separate hook for each post-scan-join processing step, but that seems messy and not clearly more useful.) Following our time-honored tradition, there's no documentation for this hook outside the source code. As-is, this hook is only meant for custom scan providers, which we can't assume very much about. A followon patch will implement an FDW callback to let FDWs do the same thing in a somewhat more structured fashion.
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