Postgres: does not support DROP FUNCTION

[zidaye]

fn test() {
    let sql = "DROP FUNCTION IF EXISTS test_func";

    let dialect = PostgreSqlDialect {};

    let ast = Parser::parse_sql(&dialect, sql).unwrap();

    println!("AST: {:?}", ast);
}

The result after the test case is executed：

panicked at 'called `Result::unwrap()` on an `Err` value: ParserError("Expected TABLE, VIEW, INDEX, ROLE, SCHEMA, or SEQUENCE after DROP, found: FUNCTION")', examples/parse_select.rs:23:48

[zidaye]

pr #752