What is a visibility map?
(definition) Definition: A planar subdivision that encodes the visibility information, that is, which points are mutually visible. See also horizontal visibility map, vertical visibility map.
What is visibility map in Postgres?
PostgreSQL tables contain visibility maps (VM), which provide information about which blocks in the table are known to contain only tuples that are visible to all transactions. For such a block, index-only scans need not physically visit the block to confirm that a TID obtained from the index is valid.