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A tablespace is a directory that can be outside the PGDATA directory and can also belong to different storage. Tablespaces are mapped into the PGDATA directory by means of symbolic links stored in the pg_tblspc subdirectory. In this way, the PostgreSQL processes do not have to look outside PGDATA, but are still able to access “external” storage.
A tablespace can be used to achieve different aims, such as enlarging the storage data or providing different storage performances for specific objects. For instance, you can create a tablespace on a slow disk to contain infrequently accessed objects and tables, keeping fast storage within another tablespace for frequently accessed objects.
You don’t have to make links by yourself: PostgreSQL provides the TABLESPACE feature to manage this and the cluster will create and manage the appropriate links under the pg_tblspc subdirectory.
$ ls -l /postgres/16/data/pg_tblspc/
$ oid2name -s
- pg_default
the default tablespace corresponding to “none,” the default storage to be used for every object when nothing is explicitly specified. In other words, every object stored directly under the PGDATA directory is attached to the pg_default tablespace.
- pg_global
the tablespace used for system-wide objects.