Makes adjacent image queries saner (i.e. doesn't query ALL OF THEM). Useful if you have a huge number of images attached to a post.

sane-adjacent-images.php

<?php
/*
Plugin Name: Sane Adjacent Images queries
Description: Makes adjacent image queries saner if you have a huge number of images attached to a post.
Version: 0.2
Author: Mark Jaquith
Author URI: http://coveredwebservices.com/
*/

class CWS_Sane_Adjacent_Images {
	static $instance;
	private $cache = array();

	public function __construct() {
		self::$instance = $this;
		add_action( 'init', array( $this, 'init' ) );
	}

	public function init() {
		add_action( 'parse_query', array( $this, 'parse_query' ) );
	}

	protected function set( $id, $trio ) {
		$this->cache[ absint( $id ) ] = $trio;
	}

	protected function get( $id ) {
		$id = absint( $id ) ;
		if ( isset( $this->cache[ $id ] ) )
			return $this->cache[ $id ];
		else
			return false;
	}

	public function parse_query( $q ) {
		global $wpdb;
		$vars = &$q->query_vars;
		if (
			isset( $vars['post_parent'] ) &&
			$vars['post_parent'] &&
			isset( $vars['orderby'] ) &&
			'menu_order ID' == $vars['orderby'] &&
			isset( $vars['post_type'] ) &&
			'attachment' == $vars['post_type'] &&
			isset( $vars['post_mime_type'] ) &&
			'image' == $vars['post_mime_type'] &&
			isset( $vars['numberposts'] ) &&
			-1 == $vars['numberposts']
			) {
				// Backtrace to make sure we're only using this when called from adjacent_image_link()
				$backtrace = debug_backtrace();
				if ( !isset( $backtrace[8]['function'] ) || 'adjacent_image_link' !== $backtrace[8]['function'] )
					return;
				$post = get_post();
				if ( $post ) {
					if ( ! $includes = $this->get( $post->ID ) ) {
						$includes = array();
						$query = "SELECT ID FROM {$wpdb->posts} WHERE {$wpdb->posts}.post_parent = %d AND {$wpdb->posts}.post_type = 'attachment' AND {$wpdb->posts}.post_mime_type LIKE %s AND ( {$wpdb->posts}.menu_order %%DIRECTION%% %d OR {$wpdb->posts}.ID %%DIRECTION%% %d ) ORDER BY {$wpdb->posts}.menu_order, {$wpdb->posts}.ID ASC LIMIT 1";
						foreach ( array( '>', '<' ) as $direction ) {
							// var_dump( $wpdb->prepare( str_replace( '%%DIRECTION%%', $direction, $query ), $vars['post_parent'], 'image/%', $post->menu_order, $post->ID ) );
							$result = $wpdb->get_var( $wpdb->prepare( str_replace( '%%DIRECTION%%', $direction, $query ), $vars['post_parent'], 'image/%', $post->menu_order, $post->ID ) );
							if ( is_numeric( $result ) )
								$includes[] = absint( $result );
						}
						// var_dump( $includes );
						$this->set( $post->ID, $includes );
					}
					// Always add the "center" attachment
					$includes[] = absint( $post->ID );

					// Now we limit the query to just have the "center" attachment and 0-2 adjacent posts
					$q->set( 'post__in', $includes );
				}
		}
	}

}

new CWS_Sane_Adjacent_Images;