WordPress query single post by slug

How about?

<?php
   $queried_post = get_page_by_path('my_slug',OBJECT,'post');
?>

From the WordPress Codex:

<?php
$the_slug = 'my_slug';
$args = array(
  'name'        => $the_slug,
  'post_type'   => 'post',
  'post_status' => 'publish',
  'numberposts' => 1
);
$my_posts = get_posts($args);
if( $my_posts ) :
  echo 'ID on the first post found ' . $my_posts[0]->ID;
endif;
?>

WordPress Codex Get Posts

a less expensive and reusable method

function get_post_id_by_name( $post_name, $post_type = 'post' )
{
    $post_ids = get_posts(array
    (
        'post_name'   => $post_name,
        'post_type'   => $post_type,
        'numberposts' => 1,
        'fields' => 'ids'
    ));

    return array_shift( $post_ids );
}