Example migrated from Codex:

To include the required tables in the query use the posts_join filter.

The below example adds a meta field for use in displaying search results.

// Join for searching metadata
function AIOThemes_joinPOSTMETA_to_WPQuery($join) {
    global $wp_query, $wpdb;

    if (!empty($wp_query->query_vars['s'])) {
        $join .= "LEFT JOIN $wpdb->postmeta ON $wpdb->posts.ID = $wpdb->postmeta.post_id ";
    }

    return $join;
}

add_filter('posts_join', 'AIOThemes_joinPOSTMETA_to_WPQuery');

And then the specific search:

function AIO_AlphabeticSearch_WhereString( $where, &$wp_query )
{
    global $wpdb;
    if(isset($_GET['aioAlphaSearchMode']) && $_GET['aioAlphaSearchMode'] == 1){

        $searchAlphabet = esc_sql($_GET['s']); 

        $where .= ' AND ' . $wpdb->posts . '.post_title LIKE \''.$searchAlphabet.'%\' ';

        // use only if the post meta db table has been joined to the search tables using posts_join filter
        $where .= " AND ($wpdb->postmeta.meta_key = 'JDReview_CustomFields_ReivewOrNewsPostType' AND $wpdb->postmeta.meta_value = 'JDReview_PostType_ReviewPost') ";
        return $where;
    }
}

add_filter( 'posts_where', 'AIO_AlphabeticSearch_WhereString', 10, 2 );