How to count post meta key values for all posts in database

In my post meta data I have the following ‘key’ for every post:
“image_details” a typical value of which is an array such as:

a:3:{s:23:"info_window_bg_image_id";s:3:"128";s:12:"colour_start";s:7:"#000000";s:10:"colour_end";s:7:"#b7b7b7";}

In the above example – info_window_bg_image_id is equal to 128

Ideally I would like to find out how many posts have the image_details['info_window_bg_image_id'] set to 128 but I would settle to find out if any of the posts have that value set.

The only way that I can think to do it (with my limited knowledge) is something along the lines of:

1. Retrieve an array of every post in the database

2. Foreach through the array using the ID’s

3. Use get_post_meta(ID, Key) to get the “image_details” key

4. Check the value of the ‘info_window_bg_image_id’ element of the result to see if it matches the value.

This seems like it would be a heavy load because I would have to do this multiple times to check for other values.

Is there an easier way of doing this? Perhaps with a direct SQL query via the WP_Query class?

Just to reiterate my ideal would be to get a result that lets me know there are:

• 6 posts have post_meta image_details ‘info_window_bg_image_id’ set to
  128
• 10 posts have post_meta image_details ‘info_window_bg_image_id’ set
  to 127
• 0 posts have post_meta image_details ‘info_window_bg_image_id’ set to
  126

etc etc

But I would also be happy with a result that just says:

128 is used, 127 is used, 126 isn’t used etc