Wordpress: Also search tags in addition to post title and content

functions.php

function tax_search_join( $join, $query ) {
    global $wpdb;
    if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
        $join .= "
            INNER JOIN
                {$wpdb->term_relationships} ON {$wpdb->posts}.ID = {$wpdb->term_relationships}.object_id
            INNER JOIN
                {$wpdb->term_taxonomy} ON {$wpdb->term_taxonomy}.term_taxonomy_id = {$wpdb->term_relationships}.term_taxonomy_id
            INNER JOIN
                {$wpdb->terms} ON {$wpdb->terms}.term_id = {$wpdb->term_taxonomy}.term_id
        ";
    }
    return $join;
}

function tax_search_where( $where, $query ) {
    global $wpdb;
    if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
        $where .= $wpdb->prepare( " OR (
            {$wpdb->term_taxonomy}.taxonomy LIKE 'post_tag'
            AND
            {$wpdb->terms}.name LIKE ('%%%s%%')
        ) ", get_query_var('s'));
    }
    return $where;
}

function tax_search_groupby( $groupby, $query ) {
    global $wpdb;
    if ( ! is_admin() && $query->is_main_query() && $query->is_search() ) {
        $groupby = "{$wpdb->posts}.ID";
    }
    return $groupby;
}

add_filter('posts_join', 'tax_search_join', 10, 2);
add_filter('posts_where', 'tax_search_where', 10, 2);
add_filter('posts_groupby', 'tax_search_groupby', 10, 2);

Based on https://stackoverflow.com/a/13493126/167378