ALTER SYSTEM — change a server configuration parameter
ALTER SYSTEM SETconfiguration_parameter{ TO | = } {value| 'value' | DEFAULT } ALTER SYSTEM RESETconfiguration_parameterALTER SYSTEM RESET ALL
   ALTER SYSTEM is used for changing server configuration
   parameters across the entire database cluster.  It can be more convenient
   than the traditional method of manually editing
   the postgresql.conf file.
   ALTER SYSTEM writes the given parameter setting to
   the postgresql.auto.conf file, which is read in
   addition to postgresql.conf.
   Setting a parameter to DEFAULT, or using the
   RESET variant, removes that configuration entry from the
   postgresql.auto.conf file. Use RESET
   ALL to remove all such configuration entries.
  
   Values set with ALTER SYSTEM will be effective after
   the next server configuration reload, or after the next server restart
   in the case of parameters that can only be changed at server start.
   A server configuration reload can be commanded by calling the SQL
   function pg_reload_conf(), running pg_ctl reload,
   or sending a SIGHUP signal to the main server process.
  
   Only superusers can use ALTER SYSTEM.  Also, since
   this command acts directly on the file system and cannot be rolled back,
   it is not allowed inside a transaction block or function.
  
configuration_parameterName of a settable configuration parameter. Available parameters are documented in Chapter 19.
value
      New value of the parameter.  Values can be specified as string
      constants, identifiers, numbers, or comma-separated lists of
      these, as appropriate for the particular parameter.
      DEFAULT can be written to specify removing the
      parameter and its value from postgresql.auto.conf.
     
   This command can't be used to set data_directory,
   nor parameters that are not allowed in postgresql.conf
   (e.g., preset options).
  
See Section 19.1 for other ways to set the parameters.
   Set the wal_level:
ALTER SYSTEM SET wal_level = replica;
   Undo that, restoring whatever setting was effective
   in postgresql.conf:
ALTER SYSTEM RESET wal_level;
   The ALTER SYSTEM statement is a
   PostgreSQL extension.