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Commitb5a9b18cintroduced block streaming infrastructure with a special fast path for all-cached scans, and commitb7b0f3f2connected the infrastructure up to sequential scans. One of the fast path micro-optimizations had an unintended consequence: it interfered with parallel sequential scan's block range allocator (from commit56788d21), which has its own ramp-up and ramp-down algorithm when handing out groups of pages to workers. A scan of an all-cached table could give extra blocks to one worker, when others had finished. In some plans (probably already very bad plans, such as the one reported by Alexander), the unfairness could be magnified. An internal buffer of 16 block numbers is removed, keeping just a single block buffer for technical reasons. Back-patch to 17. Reported-by: Alexander Lakhin <exclusion@gmail.com> Discussion: https://postgr.es/m/63a63690-dd92-c809-0b47-af05459e95d1%40gmail.com