Hi, On 2024-12-02 11:07:38 -0500, Peter Geoghegan wrote: > Clearly it would be wildly impractical to do the "buffer pin interlock > against TID recycling" thing in the context of bitmap scans.
That's certainly true if we do the VM check at the time of the bitmap heap scan. What if we did it whenever we first enter a block into the TID bitmap? There's sufficient padding space in PagetableEntry to store such state without increasing memory usage. That'd probably need some API evolution, because we'd only know after entering into the tidbitmap that we need to check the VM, we'd need to index pins during the VM checking and then update PagetableEntry with the result of the VM probe. I think, but am not certain, that it'd be sufficient to do the VM check the first time an index scan encounters a block. If a concurrent vacuum would mark the heap page all-visible after we encountered it first, we'd do "real" visibility checks, because the first VM lookup won't have it as all visible. And in the opposite situation, where we find a page all-visible in the VM, which then subsequently gets marked not-all-visible, normal snapshot semantics would make it safe to still consider the contents all-visible, because update/delete can't be visible to "us". Greetings, Andres Freund
