How metadata API works?

Update 2: Check out @NateWr method using add_post_meta with the default post:

add_post_meta( 1, 'my_items', 1 );
add_post_meta( 1, 'my_items', 2 );
add_post_meta( 1, 'my_items', new \stdClass );  

var_dump(get_post_meta(1, 'my_items', true));
echo "<br><br>";
var_dump(get_post_meta(1));
echo "<br><br>";

$GLOBALS['wp_object_cache']->stats();

And the results:

string(1) "1" 

array(1) {
  ["my_items"]=>array(3) {
    [0]=> string(1) "1"
    [1]=> string(1) "2"
    [2]=> string(19) "O:8:"stdClass":0:{}"
  }
}

This time, it hits the cache but the meta value is different. Now, if I don’t know meta_key and I just use only post ID to retrieve and parse post meta manually, it’s obvious that i will get wrong value.

Update 1: Check out @NateWr method using update_post_meta with the default post:

update_post_meta(1, 'my_items', 1);
update_post_meta(1, 'my_items', 2);
update_post_meta(1, 'my_items', 3);

var_dump( get_post_meta(1) );

echo "<br><br>";

$GLOBALS['wp_object_cache']->stats();

Results:

array(1) {
  ["my_items"]=>array(1) {
    [0]=>string(1) "3"
  }
} 

Look like Cache Hits doesn’t change and WordPress still return an meaningless array as meta_value. Or I missed something?

Question:

I’m trying to avoid extra queries on templates by pre-fetching necessary data while the WP_Query retrieving posts. This is basically how it looks like:

add_filter('the_posts', function($posts, $query)
{
  if ( !is_admin() ) {
    $mapper = new Vendor\PostMapper($GLOBALS['wpdb']);
    foreach ($posts as $post) {
      $post->post_link = get_permalink($post);
      $post->post_meta = $mapper->getMetadata($post);
      $post->post_thumbnail = $mapper->getThumbnail($post);
      ...
    }
  }

  return $posts;
}, PHP_INT_MAX, 2);

Everything works fine but I feel unease while parsing metadata. By using metadata API, if I don’t specify meta_key, meta values are always arrays of only one element such as:

array(3) {
  ["_edit_last"]=>array(1) {
    [0]=>string(1) "1"
  }
  ["_edit_lock"]=>array(1) {
    [0]=>string(12) "1467358332:1"
  }
  ["_thumbnail_id"]=>array(1) {
    [0]=>string(3) "767"
  }
}

I think I will never be able to know all meta_key. I also have no ideas why WordPress need to do that. The arrays look meaningless and it bothers me a lot.

So, given that meta_key isn’t required and I don’t specify meta_key, I really need your help to understand:

Q1: Can one meta_key have multiple meta_value? If not, why WordPress need to use an meaningless array as output for meta_value?

Q2: Why WordPress don’t unserialize all serialized meta values?

Q1
Yes, you can store serialized arrays as a single meta key, eg:

update_post_meta( $post_id, 'my_items', array( 1, 2, 3, 4 );

Furthermore, you can store multiple values against a single meta key, eg:

add_post_meta( $post_id, 'my_items', 1 );
add_post_meta( $post_id, 'my_items', 2 );

The latter is preferred because meta queries can be more efficiently performed against them.

In the first example, there is a single row in the post_meta table. In the second example, there are two rows.

So, in the second example, you can perform a query for posts which have a my_item meta value of 2 without having to pull out and unserialize every my_item entry in the database.

Many people, myself included, learned this the hard way.

Q2
I’m sorry I can’t answer the why, though I do know that serialization and unserialization can have performance implications for large data sets.