On Tue, Jul 9, 2019 at 10:15 PM Jean Louis <bugs@gnu.support> wrote: > * Konstantin Malanchev <hombit@xxxxxxxxx> [2019-07-09 12:10]: > > I have 8 GB RAM and /dev/shm size is 4GB, and there is no significant memory usage by other system processes. I surprised that Postgres uses more space in /dev/shm than sharred_buffers parameter allows, probably I don't understand what this parameter means. > > > > I have no opportunity to enlarge total RAM and probably this query requires too much RAM to execute. Should Postgres just use HDD as temporary storage in this case? > > That I cannot know. I know that /dev/shm could > grow as much as available free RAM. Hi, PostgreSQL creates segments in /dev/shm for parallel queries (via shm_open()), not for shared buffers. The amount used is controlled by work_mem. Queries can use up to work_mem for each node you see in the EXPLAIN plan, and for each process, so it can be quite a lot if you have lots of parallel worker processes and/or lots of tables/partitions being sorted or hashed in your query. -- Thomas Munro https://enterprisedb.com
