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The foreign-key-aware logic for estimation of join sizes (added in commit 100340e2d) blindly tried to apply the concept to rels that are actually parents of inheritance trees. This is just plain wrong so far as the referenced relation is concerned, since the inheritance scan may well produce lots of rows that are not participating in the constraint. It's wrong for the referencing relation too, for the same reason; although on that end we could conceivably detect whether all members of the inheritance tree have equivalent FK constraints pointing to the same referenced rel, and then proceed more or less as we do now. But pending somebody writing code to do that, we must disable this, because it's producing completely silly estimates when there's an FK linking the heads of inheritance trees. Per bug #14404 from Clinton Adams. Back-patch to 9.6 where the new estimation logic came in. Report: <20161028200412.15987.96482@wrigleys.postgresql.org>
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