tag (
  id
  tag
  raw_tag
)

object_tag (
  tag_id
  object_id
)


exemple :

[tag]
1 | truc | Truc
2 | truc | truc
3 | truc | TRUC
4 | bidule | Bidule
5 | bidule | BIDULE

[object_tag]
1  | 1
2  | 1
3  | 2
4  | 1
5  | 3
6  | 3
7  | 4
8  | 4
9  | 5
10 | 1
11 | 2
12 | 3

---------------------------

SELECT 	
	DISTINCT (tag.tag) AS tag, 
	COUNT (tag)
FROM	tag
JOIN   object_tag
	ON (tag.id = object_tag.tag_id)
GROUP BY tag
ORDER BY COUNT (tag) DESC
OFFSET 0
LIMIT 20