Fast Next/Prev Post Links for large databases. http://wordpress.stackexchange.com/q/101435/

gistfile1.php

<?php # -*- coding: utf-8 -*-
namespace WPSE;
/* Plugin Name: Fast Next/Prev Post Links
 * Plugin URI:  http://wordpress.stackexchange.com/q/101435/
 */

add_action(
    'fast_prev_post_link',
    __NAMESPACE__ . '\fast_prev_post_link',
    10,
    4
);
add_action(
    'fast_next_post_link',
    __NAMESPACE__ . '\fast_next_post_link',
    10,
    4
);
add_action(
    'transition_post_status',
    __NAMESPACE__ . '\delete_fast_adjacent_meta',
    10,
    3
);


/**
 * Print previous post link.
 *
 * @param unknown_type $format
 * @param unknown_type $link
 * @param unknown_type $in_same_cat
 * @param unknown_type $excluded_categories
 */
function fast_prev_post_link(
    $format              = '&laquo; %link',
    $link                = '%title',
    $in_same_cat         = FALSE,
    $excluded_categories = ''
    )
{
    empty ( $format ) && $format = '%link &raquo;';
    fast_adjacent_post_link(
        $format,
        $link,
        $in_same_cat,
        $excluded_categories,
        TRUE
    );

}
/**
 * Print next post link.
 *
 * @param  string $format
 * @param  string $link
 * @param  bool $in_same_cat
 * @param  array|string $excluded_categories
 * @return void
 */
function fast_next_post_link(
    $format              = '%link &raquo;',
    $link                = '%title',
    $in_same_cat         = FALSE,
    $excluded_categories = ''
    )
{
    empty ( $format ) && $format = '%link &raquo;';

    fast_adjacent_post_link(
        $format,
        $link,
        $in_same_cat,
        $excluded_categories,
        FALSE
    );
}

/**
 * Display adjacent post link.
 *
 * Slightly changed copy of adjacent_post_link().
 * Unfortunately, WP mixes retrieving data and display. :(
 *
 * Can be either next post link or previous.
 *
 * @param  string       $format              Link anchor format.
 * @param  string       $link                Link permalink format.
 * @param  bool         $in_same_cat         Whether link should be in a same
 *                                           category.
 * @param  array|string $excluded_categories Array or comma-separated list of
 *                                           excluded category IDs.
 * @param  bool         $previous            Default is true. Whether to display
 *                                           link to previous or next post.
 * @return void
 */
function fast_adjacent_post_link(
    $format,
    $link,
    $in_same_cat         = FALSE,
    $excluded_categories = '',
    $previous            = TRUE
    )
{
    if ( $previous && is_attachment() )
        $post = get_post( get_post()->post_parent );
    else // the only real change
        $post = get_fast_adjacent_post(compact(
            'in_same_cat',
            'excluded_categories',
            'previous'
        ));

    if ( ! $post ) {
        $output = '';
    } else {
        $title = $post->post_title;

        if ( empty( $post->post_title ) )
            $title = $previous ? __( 'Previous Post' ) : __( 'Next Post' );

        $title = apply_filters( 'the_title', $title, $post->ID );
        $date = mysql2date( get_option( 'date_format' ), $post->post_date );
        $rel = $previous ? 'prev' : 'next';

        $string = '<a href="' . get_permalink( $post ) . '" rel="'.$rel.'">';
        $inlink = str_replace( '%title', $title, $link );
        $inlink = str_replace( '%date', $date, $inlink );
        $inlink = $string . $inlink . '</a>';

        $output = str_replace( '%link', $inlink, $format );
    }

    $adjacent = $previous ? 'previous' : 'next';

    echo apply_filters( "{$adjacent}_post_link", $output, $format, $link, $post );
}

/**
 * Get next or previous post from post meta.
 *
 * @param array $args Optional, The defaults are as follows:
 *    bool          $in_same_cat          = false
 *    string|array  $excluded_categories  = array()
 *    unknown_type  $previous             = true
 *    object        $post                 = null
 */
function get_fast_adjacent_post( $args = array() )
{

    extract(shortcode_atts(array(
        'in_same_cat'         => false,
        'excluded_categories' => '',
        'previous'            => true,
        'post'                => null
    ), $args));

    if ( ! $post = get_post( $post ) )
        return;

    $excluded_categories = prepare_excluded_categories( $excluded_categories );

    $type = $previous ? 'prev' : 'next';
    $cat_hash = empty ( $excluded_categories ) ? 0 : join( '-', $excluded_categories );
    $hash = (int) $in_same_cat . "-$cat_hash";

    $meta = (array) get_post_meta( $post->ID, "_fast_{$type}_post", TRUE );

    if ( isset ( $meta[ $hash ] ) )
        return get_post( $meta[ $hash ] );

    $adjacent_post = get_adjacent_post( $in_same_cat, $excluded_categories, $previous );
    $meta[ $hash ] = $adjacent_post->ID;
    update_post_meta( $post->ID, "_fast_{$type}_post", $meta );

    return get_post( $meta[ $hash ] ) ;
}

/**
 * Prepare categories sent as string.
 *
 * @param  string|array $cats
 * @return array
 */
function prepare_excluded_categories( $cats )
{
    if ( empty ( $cats ) or is_array( $cats ) )
        return array();

    $cats = explode( ',', $cats );
    $cats = array_map( 'trim', $excluded_categories );
    $cats = array_map( 'intval', $excluded_categories );

    return $cats;
}

/**
 * Deletes post meta values for the current post and all posts referencing it.
 *
 * @wp-hook transition_post_status
 * @param   string $new_status
 * @param   string $old_status
 * @param   object $post
 * @return  void
 */
function delete_fast_adjacent_meta( $new_status, $old_status, $post )
{
    $prev = (array) get_post_meta( $post->ID, '_fast_prev_post', TRUE );

    if ( ! empty ( $prev ) )
    {
        foreach ( $prev as $post_id )
            delete_post_meta( $post_id, '_fast_next_post' );
    }

    $next = (array) get_post_meta( $post->ID, '_fast_next_post', TRUE );

    if ( ! empty ( $next ) )
    {
        foreach ( $next as $post_id )
            delete_post_meta( $post_id, '_fast_prev_post' );
    }

    delete_post_meta( $post->ID, '_fast_prev_post' );
    delete_post_meta( $post->ID, '_fast_next_post' );
}

function get_fast_previous_post( $args = array() )
{
    $args['previous'] = true;
    return get_fast_adjacent_post($args);
}

function get_fast_next_post( $args = array() )
{
    $args['previous'] = false;
    return get_fast_adjacent_post($args);
}


// To print the links, use this in the template:
//   do_action( 'fast_prev_post_link' );
//   do_action( 'fast_next_post_link' );
//
// To get the posts, use:
//   $in_same_cat = 1;
//   WPSE\get_fast_previous_post( compact(in_same_cat) );
//   WPSE\get_fast_next_post(     compact(in_same_cat) );