WP_query fields
<?php
// Create a new Query
$args = array(
'post_type' => 'post',
'cat' => '1,6,8',
'author_name' => 'john',
'posts_per_page' => 5,
'fields' => 'ids'
);
$the_query = new WP_Query( $args );
// The Loop
echo '<ul>':
while ( $the_query->have_posts() ):
$the_query->the_post();
echo '<li>' . get_the_ID() . '</li>';
endwhile;
echo '</ul>':
// Restore original Query
wp_reset_query();
?>