include_if_exists is a configuration file directive specifying the name of another configuration file to include, if that file exists. No error is raised if the file does not exist.
include_if_exists was added in PostgreSQL 9.2.
The specified file can be either relative to the current configuration file, or an absolute file path.
include_if_exists directive used with a non-existent file:
... include_if_exists 'foo.conf' ...
After reloading the configuration, e.g. with
pg_reload_conf(), following log line will be emitted:
[2020-05-02 13:29:26 UTC] node2 postgres [unknown] LOG: 00000: skipping missing configuration file "/var/lib/pgsql/data/foo.conf"