Visibility map & index-only scans
Simple terms
Every table page carries a bit meaning every row here is visible to everyone. That bit is what makes an index-only scan possible: the index already holds your answer, and the bit is permission to trust it without opening the table. VACUUM sets the bit. Any write clears it. So an index-only scan is a privilege, not a property, leave a long transaction open, VACUUM can't set the bits, and Heap Fetches climbs in EXPLAIN while your fast plan quietly goes back to touching the heap.
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What is the visibility map, what is the 'Heap Fetches' line in an Index Only Scan, and why can a long-running transaction silently turn a fast index-only scan back into one that visits the heap?
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