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Question

I need to have pagination only for posts marked as sticky. No non-sticky posts need to be shown.

I had this in the functions.php file

function sticky_pagination()
{
    //global $wp_query;
    $big = 999999999;

    $args = array('post_type' => 'post', 'posts_per_page' => 5, 'post__in'  => get_option( 'sticky_posts' ));

    $wp_query = new WP_Query($args);

    echo paginate_links(array(
        'base'    => str_replace($big, '%#%', get_pagenum_link($big)),
        'format'  => '?paged=%#%',
        'current' => max(1, get_query_var('paged')),
        'total'   => $wp_query->max_num_pages
    ));

    wp_reset_postdata();
}

But pagination doesnt work right and I get a headers already sent error on /page/1/ but on the posts index page it doesnt do that and on /page/2/ it doesnt give me that either. Plus it dumps out an extra echo of the prev/next which is wierd.

Answer 1

Querying only certain sticky posts or trying to page a list of sticky posts can be tricky if you do not know how sticky posts work within the WP_Query class.

Lets quickly look at how stickies are queried inside the WP_Query class

// Put sticky posts at the top of the posts array
$sticky_posts = get_option('sticky_posts');
if ( $this->is_home && $page <= 1 && is_array($sticky_posts) && !empty($sticky_posts) && !$q['ignore_sticky_posts'] ) {
    $num_posts = count($this->posts);
    $sticky_offset = 0;
    // Loop over posts and relocate stickies to the front.
    for ( $i = 0; $i < $num_posts; $i++ ) {
        if ( in_array($this->posts[$i]->ID, $sticky_posts) ) {
            $sticky_post = $this->posts[$i];
            // Remove sticky from current position
            array_splice($this->posts, $i, 1);
            // Move to front, after other stickies
            array_splice($this->posts, $sticky_offset, 0, array($sticky_post));
            // Increment the sticky offset. The next sticky will be placed at this offset.
            $sticky_offset++;
            // Remove post from sticky posts array
            $offset = array_search($sticky_post->ID, $sticky_posts);
            unset( $sticky_posts[$offset] );
        }
    }
    // If any posts have been excluded specifically, Ignore those that are sticky.
    if ( !empty($sticky_posts) && !empty($q['post__not_in']) )
        $sticky_posts = array_diff($sticky_posts, $q['post__not_in']);
    // Fetch sticky posts that weren't in the query results
    if ( !empty($sticky_posts) ) {
        $stickies = get_posts( array(
            'post__in' => $sticky_posts,
            'post_type' => $post_type,
            'post_status' => 'publish',
            'nopaging' => true
        ) );
        foreach ( $stickies as $sticky_post ) {
            array_splice( $this->posts, $sticky_offset, 0, array( $sticky_post ) );
            $sticky_offset++;
        }
    }
}

As you can see, if we pass the correct parameters to WP_Query and is_home() returns true, WP_Query will return all sticky posts regardless. This happens when you use the post__in parameter. What this means is, if you only query sticky posts conditionally, you will always get all sticky posts back on page one regardless of what you passed as arguments. To stop this from happening, we need to set ignore_sticky_posts to 1 in order for us to avoid querying all sticky posts.

Just on the note of post__in . You should always make sure you have a valid array of post id's before passing anything to post__in . If you pass an empty array, like when there is no sticky posts, all posts are returned regardless, it does not return an empty array as one would expect, so be careful of this

Lets look at a possible soltion ( Untested and requires PHP 5.4+ )

// Get all sticky posts
$stickies = get_option( 'sticky_posts' );
// Make sure we have sticky posts
if ( $stickies ) {
    // We have stickies, lets set our query
    $args = [
        'post__in'            => $stickies,
        'ignore_sticky_posts' => 1,
        'posts_per_page'      => 5,
        'paged'               => get_query_var( 'paged', 1 ),
        // Any other arguments
    ];
    $q = new WP_Query( $args );

    // YOUR LOOP
    while ( $q->have_posts() ) {
        $q->the_post();

            // YOUR OUTPUT

    }

    /**
     * Lets use my pagination function 
     * @link http://wordpress.stackexchange.com/a/172818/31545
     */
    if ( function_exists( '' ) ) {
        $paging_args = [
            'query'            => $q
        ];
        echo get_paginated_numbers( $paging_args );
    }

    wp_reset_postdata();
}