EXPLAIN
SELECT
    posts.*
FROM
    posts
    INNER JOIN categories ON posts.category_id = categories.id
        AND categories.main = 1
        AND(categories.private_category = 0
            OR categories.private_category IS NULL)
WHERE
    posts.id NOT IN('')
    AND posts.deleted = 0
    AND posts.hidden = 0
    AND posts.total_points >= - 5
ORDER BY
    posts.id DESC
LIMIT 25;

| id | select_type | table       | partitions | type | possible_keys                                                                                                                                                    | key                         | key_len | ref                           | rows  | filtered | Extra                                        |
|----|-------------|-------------|------------|------|------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------|---------|-------------------------------|-------|----------|----------------------------------------------|
|  1 | SIMPLE      | categories  |            | ALL  | PRIMARY,index_categories_on_private_category                                                                                                                     |                             |         |                               |    12 |    10.00 | Using where; Using temporary; Using filesort |
|  1 | SIMPLE      | posts       |            | ref  | PRIMARY,index_posts_on_category_id,index_posts_on_deleted_and_hidden_and_user_id_and_created_at,index_posts_deleted,index_posts_hidden,index_posts_total_points  | index_posts_on_category_id  | 5       | mydb.categories.id            | 37516 |    12.50 | Using index condition; Using where           |

| id | select_type | table       | type   | possible_keys                                                                                                                                                    | key     | key_len | ref                               | rows  | Extra       |
|----|-------------|-------------|--------|------------------------------------------------------------------------------------------------------------------------------------------------------------------|---------|---------|-----------------------------------|-------|-------------|
|  1 | SIMPLE      | posts       | range  | PRIMARY,index_posts_on_category_id,index_posts_on_deleted_and_hidden_and_user_id_and_created_at,index_posts_deleted,index_posts_hidden,index_posts_total_points  | PRIMARY | 4       |                                   | 37516 | Using where |
|  1 | SIMPLE      | categories  | eq_ref | PRIMARY,index_categories_on_private_category,index_categories_on_main                                                                                            | PRIMARY | 4       | mydb.posts.category_id            |    12 | Using where |