Finding posts with exactly set of tags per user

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Question :

I’m trying to determine the best way to find all posts that have an exact match with a list of user tags. For example, user has tag IDs: 1,5,9, and 11, while post has tag IDs 1, 5 and 9, thus this is selected as a valid post to display to the user.

The only way I could think of to do this is to count the number of unique tags for each post then count the number of tags a user has in common with the post, compare two values, if equal then it is a match (see query below). Is this truly the best way?


Posts_tags (tag_id, post_id) unique enforced
Users_tags (tag_id, user_id) unique enforced

Current query (broken down so more legible):
Sub-Query #1:

SELECT     COUNT(*) as numrows, pt.post_id AS id
FROM       users_tags AS ut
INNER JOIN posts_tags AS pt
ON         ut.tag_id = pt.tag_id
WHERE      ut.user_id = 1 
GROUP BY   pt.post_id

Sub-Query #2:

SELECT   COUNT(*) AS numrows, post_id AS id
FROM     posts_tags 
GROUP BY post_id

Master Query:

FROM       (Sub-Query #1) t1 
INNER JOIN (Sub-Query #2) t2 
ON         (t1.numrows = t2.numrows)
AND        ( =

Also, would a graph database like neo4j be better at this type of query?

Answer :

If you are using MySQL 5.6 or later, then the query will be reasonably efficient. If using an older version, it is “order N*N”, which is very slow for large subqueries.

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