Disable (or limit) queries when certain content (or data) is not needed (or showed)

Question

Currently I am making a custom design for a site. The problem I'm seeing in Wordpress is about almost all data pulled out from the database.

While in certain pages - like front-page.php - it only needs, for example, title, category, two metas and the excerpt, WP gets everything. Hell, the post content is only visible in post-single.php, but instead WP pulls out all the content, tags, guid, tags, etc. where it can (print_r helps a lot).

The question is about how to limit Wordpress data pulled out from his queries. I think that I should make a net "WP_Query" (or override it) based on $wpdb and use it for the site, instead of using the default one. Well, at least functions like query_posts, WP_Query and get_posts doesn't allow me to limit the data pulled out.

I am also interested in caching the data, because is really small, but this is first.

Update 1: Tried to "minimize data":

    add_action( 'pre_get_posts', 'disable_main_query' );
function disable_main_query( $query ) {
    if( $query->is_main_query() && $query->is_front_page() ) {
        add_filter('posts_fields', 'minimized_data');
    }
}

function minimized_data($fields) {
    global $wpdb;
    $fields = "
        $wpdb->posts.ID,
        $wpdb->posts.post_title,
        $wpdb->posts.post_excerpt,
        $wpdb->posts.comment_count,
        $wpdb->posts.guid,
        $wpdb->posts.post_status,
        $wpdb->posts.post_name,
        $wpdb->posts.post_date,
        $wpdb->posts.post_type
    ";
    return $fields;
}

Mow the default loop is plagued with "Undefined property: stdClass::$post_XXXX" and other things. I think the_post is to blame now, so I'm cheking.

Update 2: Has expected, it was. Changed the while loop for foreach ($posts as $post) no more undefined properties, but as earlier I can't get rid of ob_end_flush() [ref.outcontrol]: failed to delete buffer zlib output compression.

Update 3: define( 'SAVEQUERIES', FALSE ); and no more failed buffer (for WAMP).

Answer 1

You can filter 'posts_fields', check if it is the main query and limit the queried fields to … whatever you need.
See WP_Query::get_posts() in /wp-includes/query.php for details and side effects.

For debugging the queries I recommend the plugin Debug Bar. If you add …

define( 'WP_DEBUG',         TRUE );
define( 'SAVEQUERIES',      TRUE );

… to your wp-config.php you can see the resulting query and how it affects later queries.

Answer 2

After some digging, it appears that the only way to "get only what you need" is by using de class $wpdb to talk with the database. That's mean also a second loop, outside the main one.

Now, for performance sake, there should be a way to modify the main/default loop that Wordpress always does to NOT get the post content and another useless data (for the site).