Usually, administrators are surprised to find that there is no reduction in disk space from using vacuum. There is only one situation where VACUUM can actually reduce the size of a table. If the last data page of a table is empty, completely free of rows, and it's possible to obtain an exclusive lock on the table, VACUUM executes a special disk release process. It will scan backwards from the end, returning all pages it finds to the operating system as now free space, until it finds a data page that isn't empty.
Accordingly, it's only possible for a regular vacuum to reduce the size of a table if it has a contiguous chunk of free space at the end of the table. One common situation that can produce this pattern of ...
