PostgreSQL 10 High Performance - Third Edition

maintainance_work_mem

A few operations in the database server need working memory for larger operations than just regular sorting. VACUUM, CREATE INDEX, and ALTER TABLE ADD FOREIGN KEY can all allocate up to maintainance_work_mem worth of memory instead. As it's unlikely that many sessions will be doing one of these operations at once, it's possible to set this value much higher than the standard per-client work_mem setting. Note that at least autovacuum_max_workers (defaulting to 3 starting in version 8.3) will allocate this much memory, so consider those sessions (perhaps along with a session or two doing a CREATE INDEX) when setting this value.

Assuming you haven't increased the number of autovacuum workers, a typical high setting for ...