How to make an index for a query with group by and order by?

Question

MySQL

The query itself is
SELECT Ad.id, Ad.title, Ad.description, Ad.posting_date, Ad.is_colored_listing, Ad.bg_color, Ad.fg_color, `Adi`.`image_name`, `Adc`.`category_alias`
FROM `daype_ads ` AS `Ad`
LEFT JOIN daype_categories AS `Adc` ON ( `Ad`.`category_id` = `Adc`.`id` )
LEFT JOIN daype_images AS `Adi` ON ( `Ad`.`id` = `Adi` .`ad_id` )
WHERE `city_id` =25
AND `Ad`.`category` =4
AND `Ad`.`status` =1
AND `category_id` =33
GROUP BY `Ad`.`id`
ORDER BY `posting_date ` DESC
LIMIT 20, 20

Executes seconds.

Answer 1

AND `Ad`.`category` = 4
 ...
 AND `category_id` = 33

What does it mean?

Answer 2

Why grouping if there are no aggregating functions?