ALTER_SYSTEM - change a server configuration parameter
ALTER SYSTEM SET configuration_parameter { TO | = } { value | value | DEFAULT }
ALTER SYSTEM RESET configuration_parameter
ALTER SYSTEM RESET ALLALTER 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_parameter
Name of a settable configuration parameter. Available parameters are documented in Chapter 20.
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 cant be used to set data_directory, nor parameters that are not allowed in postgresql.conf (e.g., preset options).
See Section 20.1 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.