fillfactor

fillfactor is a table storage parameter: inserts stop filling a page when it is this percent full (default 100). The reserved remainder is not waste, it is the free space that lets an update stay on the same page, which is the second condition for a HOT update. On update-heavy tables a fillfactor of 90 trades 10% larger heap for a much higher HOT rate and far fewer index writes. It applies to newly written pages, so changing it needs a rewrite to affect existing data.