If you were to create a class that inherits pg.Pool like so:

const pg = require('pg');

class Db extends pg.Pool {
  insert(args) {
    return this.query('...');
  }
}

const a = new Db();

console.log(a.insert); // undefined

The method insert() would be undefined on all instances of this custom class. If you were to do the exact same thing with pg.Client, it works as expected.