MyFeature.cs

class MyFeature : FeatureSchemaBase
{
	class MyFunc : Function
	{
		public MyFunc(string schema) : base(new DbObjectName(schema, "hello"))
		{
		}

		public override void Write(DdlRules rules, StringWriter writer)
		{
			writer.WriteLine(
$@"CREATE OR REPLACE FUNCTION {Identifier}() RETURNS text AS $$
	BEGIN
			RETURN 'world';
	END;
$$ LANGUAGE plpgsql;");
		}

		protected override string toDropSql()
		{
			return $"drop function {Identifier}";
		}
	}

	public MyFeature(StoreOptions options) : base(nameof(MyFeature), options)
	{
	}

	protected override IEnumerable<ISchemaObject> schemaObjects()
	{
		yield return new MyFunc(Options.DatabaseSchemaName);
	}
}

..
var store = DocumentStore.For(c =>
{			
	c.Storage.Add<MyFeature>();
});

store.Schema.ApplyAllConfiguredChangesToDatabase();

using (var s = store.OpenSession())
{
	var str = s.Query<string>("select hello()").First();
	Console.WriteLine(str);
}