[pull] master from postgres:master #105
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While choosing an autogenerated name for an index, look for pre-existing relations using a SnapshotDirty snapshot, instead of the previous behavior that considered only committed-good pg_class rows. This allows us to detect and avoid conflicts against indexes that are still being built. It's still possible to fail due to a race condition, but the window is now just the amount of time that it takes DefineIndex to validate all its parameters, call smgrcreate(), and enter the index's pg_class row. Formerly the race window covered the entire time needed to create and fill an index, which could be very long if the table is large. Worse, if the conflicting index creation is part of a larger transaction, it wouldn't be visible till COMMIT. So this isn't a complete solution, but it should greatly ameliorate the problem, and the patch is simple enough to be back-patchable. It might at some point be useful to do the same for pg_constraint entries (cf. ChooseConstraintName, ConstraintNameExists, and related functions). However, in the absence of field complaints, I'll leave that alone for now. The relation-name test should be good enough for index-based constraints, while foreign-key constraints seem to be okay since they require exclusive locks to create. Bug: #18959 Reported-by: Maximilian Chrzan <[email protected]> Author: Tom Lane <[email protected]> Reviewed-by: Dilip Kumar <[email protected]> Discussion: https://postgr.es/m/[email protected] Backpatch-through: 13
Commit 85e5e22, which added (a forerunner of) this logic, argued that Adding the necessary complexity to make this work doesn't seem like it would be repaid in significantly better plans, because in cases where such a PHV exists, there is probably a corresponding join order constraint that would allow a good plan to be found without using the star-schema exception. The flaw in this claim is that there may be other join-order restrictions that prevent us from finding a join order that doesn't involve a "dangerous" PHV. In particular we now recognize that small join_collapse_limit or from_collapse_limit could prevent it. Therefore, let's bite the bullet and make the case work. We don't have to extend the executor's support for nestloop parameters as I thought at the time, because we can instead push the evaluation of the placeholder's expression into the left-hand input of the NestLoop node. So there's not really a lot of downside to this solution, and giving the planner more join-order flexibility should have value beyond just avoiding failure. Having said that, there surely is a nonzero risk of introducing new bugs. Since this failure mode escaped detection for ten years, such cases don't seem common enough to justify a lot of risk. Therefore, let's put this fix into master but leave the back branches alone (for now anyway). Bug: #18953 Reported-by: Alexander Lakhin <[email protected]> Diagnosed-by: Richard Guo <[email protected]> Author: Tom Lane <[email protected]> Discussion: https://postgr.es/m/[email protected]
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