Avoiding full table scan when performing inner joins in MySQL using IN in WHERE clause

Question

How can I avoid a full table scan when performing inner joins in MySQL using IN in WHERE clause? For example:

explain SELECT
-> COUNT(DISTINCT(n.nid))
-> FROM node n
-> INNER JOIN term_node tn ON n.nid = tn.nid
-> INNER JOIN content_type_article ca ON n.nid = ca.nid
-> WHERE tn.tid IN (67,100)
-> ;
+----+-------------+-------+--------+----------------------------------+---------+---------+----------------------+-------+--------------------------+
| id | select_type | table | type   | possible_keys                    | key     | key_len | ref                  | rows  | Extra                    |
+----+-------------+-------+--------+----------------------------------+---------+---------+----------------------+-------+--------------------------+
|  1 | SIMPLE      | tn    | ALL    | PRIMARY,nid                      | NULL    | NULL    | NULL                 | 42180 | Using where              |
|  1 | SIMPLE      | ca    | ref    | nid,field_article_date_nid_index | nid     | 4       | drupal_mm_qas.tn.nid |     1 | Using index              |
|  1 | SIMPLE      | n     | eq_ref | PRIMARY                          | PRIMARY | 4       | drupal_mm_qas.ca.nid |     1 | Using where; Using index |
+----+-------------+-------+--------+----------------------------------+---------+---------+----------------------+-------+--------------------------+
3 rows in set (0.00 sec)

Answer 1

It seems you're filtering by a column that mysql identified to be not selective enough. When a filter's cardinality is too low (ie, the number of distinct rows for that filter is low), mysql thinks, most of the time accurately, that a fts would be faster.

To confirm, please show the result of SELECT COUNT(DISTINCT tn.tid) FROM term_node tn and SELECT COUNT(*) FROM term_node tn