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The reason is that since index and heap scans are decoupled, there can be no assurance that the index tuple prompting a visit to a particular heap TID still exists when the visit is made. Therefore the tuple might not exist anymore either (which is OK because heap_fetch will cope) --- but worse, the tuple slot could have been re-used for a newer tuple. With an MVCC snapshot the newer tuple is certain to fail the time qual and so it will not be mistakenly returned, but with anything else we might return a tuple that doesn't meet the required index qual conditions.","Do the underlying index scan, build the bitmap, set up the parallel state needed for parallel workers to iterate through the bitmap, and set up the underlying table scan descriptor.","The leader will immediately come out of the function, but others will be blocked until leader populates the TBM and wakes them up.","Prepare to iterate over the TBM. 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The reason is that since index and heap scans are decoupled, there can be no assurance that the index tuple prompting a visit to a particular heap TID still exists when the visit is made. Therefore the tuple might not exist anymore either (which is OK because heap_fetch will cope) --- but worse, the tuple slot could have been re-used for a newer tuple. With an MVCC snapshot the newer tuple is certain to fail the time qual and so it will not be mistakenly returned, but with anything else we might return a tuple that doesn't meet the required index qual conditions.","Do the underlying index scan, build the bitmap, set up the parallel state needed for parallel workers to iterate through the bitmap, and set up the underlying table scan descriptor.","The leader will immediately come out of the function, but others will be blocked until leader populates the TBM and wakes them up.","Prepare to iterate over the TBM. 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