The pg_global tablespace is used for shared system catalogs. The pg_default tablespace is the default tablespace of the template1 and template0 databases. If the partition or volume on which the cluster was initialized runs out of space and cannot be extended, a tablespace can be created on a different partition and used until the system can be reconfigured.

An index that is heavily used can be placed on a fast, highly available disk, like a solid-state device. Also, a table storing archived data that is rarely used or not performance critical can be stored on a less expensive, slower disk system like SAS or SATA drives.

Tablespaces are a part of the database cluster and cannot be treated as an autonomous collection of data files. They depend on metadata contained in the main data directory, and therefore cannot be attached to a different database cluster or backed up individually. Similarly, if you lose a tablespace (through file deletion, disk failure, and so on), the database cluster might become unreadable or unable to start. Placing a tablespace on a temporary file system like a RAM disk risks the reliability of the entire cluster.

After it is created, a tablespace can be used from any database if the requesting user has sufficient privileges. PostgreSQL uses symbolic links to simplify the implementation of tablespaces. PostgreSQL adds a row to the pg_tablespace table (a clusterwide table) and assigns a new object identifier (OID) to that row. Finally, the server uses the OID to create a symbolic link between your cluster and the given directory. The directory $PGDATA/pg_tblspc contains symbolic links that point to each of the non-built-in tablespaces defined in the cluster.