[sgarland]

VACUUM rarely reclaims space from the OS’ perspective, if that’s what you meant. It can in certain circumstances, but they’re rare. VACUUM FULL is the equivalent to OPTIMIZE TABLE – both lock the table to do a full rewrite, and optimally binpack it to the extent that is posssible.

EDIT: my mistake, OPTIMIZE TABLE is an online DDL. I’ve been burned in the past from foreign key constraint metadata locks essentially turning it into a blocking operation.