clean_absolute_urls() function searched the wp_posts table for references to the current hostname, and removes them making all urls relative instead of absolute. This is especially useful if migrating servers or urls.

clean_absolute_urls_in_wp_posts_table.php

function find_and_replace($needle, &$haystack) {
    if(strpos($haystack, $needle) !== false) {
        $haystack = preg_replace('|' . preg_quote($needle) . '|', '', $haystack);
    }
}

/**
 * This function makes relative urls from all absolute urls of the current domain.
 *
 *
 * */
function clean_absolute_urls() {

    global $wpdb;

    $HOST_HTTP = 'http://' . $_SERVER['HTTP_HOST'];

    $HOST_HTTPS = 'https://' . $_SERVER['HTTP_HOST'];

    $failed = array();

    $success = 0;

    $posts = $wpdb->get_results("
        SELECT * FROM `wp_posts` WHERE
        `post_content` LIKE '%" .   $HOST_HTTPS     . "%' OR
        `post_content` LIKE '%" .   $HOST_HTTP      . "%' OR
        `post_excerpt` LIKE '%" .   $HOST_HTTPS     . "%' OR
        `post_excerpt` LIKE '%" .   $HOST_HTTP      . "%' OR
        `guid` LIKE '%" .           $HOST_HTTPS     . "%' OR
        `guid` LIKE '%" .           $HOST_HTTP      . "%'
        LIMIT 0,10000;
    ");

    foreach($posts as $post) {

        $content = $post->post_content;

        $excerpt = $post->post_excerpt;

        $guid = $post->guid;

        find_and_replace($HOST_HTTP, $content);

        find_and_replace($HOST_HTTPS, $content);

        find_and_replace($HOST_HTTP, $excerpt);

        find_and_replace($HOST_HTTPS, $excerpt);

        find_and_replace($HOST_HTTP, $guid);

        find_and_replace($HOST_HTTPS, $guid);

        $post->post_content = $content;
        $post->post_excerpt = $excerpt;
        $post->guid = $guid;

        $updatedID = wp_update_post($post);

        if($updatedID == 0) {
            $failed[] = $post->ID;
        } else {
            $success++;
        }
    }
    return compact('failed', 'success');
}