postgres=# \df+ pg_database_size  
                                                                                                     List of functions  
   Schema   |       Name       | Result data type | Argument data types |  Type  |  Data access   | Volatility |  Owner   | Language |      Source code      |                         Description                           
------------+------------------+------------------+---------------------+--------+----------------+------------+----------+----------+-----------------------+-------------------------------------------------------------  
 pg_catalog | pg_database_size | bigint           | name                | normal | reads sql data | volatile   | dege.zzz | internal | pg_database_size_name | Calculate total disk space usage for the specified database  
 pg_catalog | pg_database_size | bigint           | oid                 | normal | reads sql data | volatile   | dege.zzz | internal | pg_database_size_oid  | Calculate total disk space usage for the specified database  
(2 rows)