/usr/share/doc/postgresql-10/examples/insert_username.example is in postgresql-10 10.6-0ubuntu0.18.04.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | DROP TABLE username_test;
CREATE TABLE username_test (
name text,
username text not null
);
CREATE TRIGGER insert_usernames
BEFORE INSERT OR UPDATE ON username_test
FOR EACH ROW
EXECUTE PROCEDURE insert_username (username);
INSERT INTO username_test VALUES ('nothing');
INSERT INTO username_test VALUES ('null', null);
INSERT INTO username_test VALUES ('empty string', '');
INSERT INTO username_test VALUES ('space', ' ');
INSERT INTO username_test VALUES ('tab', ' ');
INSERT INTO username_test VALUES ('name', 'name');
SELECT * FROM username_test;
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