Hello

This block is reading and checking the visibility map, I think. We don't have 
to check the visibility map during the index scan - we still need to get a 
tuple from the table, we can check the visibility for current transaction 
there. With index only scan, we need to check the visibility map: if the tuple 
is visible to all transactions, then we return it. Otherwise, we read the tuple 
from the table as in the index scan (this is your case, as indicated by "Heap 
Fetches: 1")

Index only scan does not mean that we will not read the tuple from the table. 
It means that we can skip reading the table if the visibility map allows it for 
given tuple.

regards, Sergei


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