fab2af30d6
When using default autovacuum_vac_cost_limit, autovac_balance_cost relied on VacuumCostLimit to contain the correct global value ... but after the first time through in a particular worker process, it didn't, because we'd trashed it in previous iterations. Depending on the state of other autovac workers, this could result in a steady reduction of the effective cost_limit setting as a particular worker processed more and more tables, causing it to go slower and slower. Spotted by Simon Poole (bug #5759). Fix by saving and restoring the GUC variables in the loop in do_autovacuum. In passing, improve a few comments. Back-patch to 8.3 ... the cost rebalancing code has been buggy since it was put in.
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