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The premise of src/test/modules/test_plan_advice is that if we plan a query once, generate plan advice, and then replan it using that same advice, all of that advice should apply cleanly, since the settings and everything else are the same. Unfortunately, that's not the case: the test suite is the main regression tests, and concurrent activity can change the statistics on tables involved in the query, especially system catalogs. That's OK as long as it only affects costing, but in a few cases, it affects which relations appear in the final plan at all. In the buildfarm failures observed to date, this happens because we consider alternative subplans for the same portion of the query; in theory, MinMaxAggPath is vulnerable to a similar hazard. In both cases, the planner clones an entire subquery, and the clone has a different plan name, and therefore different range table identifiers, than the original. If a cost change results in flipping between one of these plans and the other, the test_plan_advice tests will fail, because the range table identifiers to which advice was applied won't even be present in the output of the second planning cycle. To fix, invent a new DO_NOT_SCAN advice tag. When generating advice, emit it for relations that should not appear in the final plan at all, because some alternative version of that relation was used instead. When DO_NOT_SCAN is supplied, disable all scan methods for that relation. To make this work, we reuse a bunch of the machinery that previously existed for the purpose of ensuring that we build the same set of relation identifiers during planning as we do from the final PlannedStmt. In the process, this commit slightly weakens the cross-check mechanism: before this commit, it would fire whenever the pg_plan_advice module was loaded, even if pg_plan_advice wasn't actually doing anything; now, it will only engage when we have some other reason to create a pgpa_planner_state. The old way was complex and didn't add much useful test coverage, so this seems like an acceptable sacrifice. Discussion: http://postgr.es/m/CA+TgmoYuWmN-00Ec5pY7zAcpSFQUQLbgAdVWGR9kOR-HM-fHrA@mail.gmail.com Reviewed-by: Lukas Fittl <lukas@fittl.com> |
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