使用WHERE子句中的IN在MySQL中执行内部联接时避免全表扫描

Question

在WHERE子句中使用IN在MySQL中执行内部联接时，如何避免全表扫描？ 例如：

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

看来您正在筛选一列mysql识别为选择性不够的列。 当一个过滤器的基数太低（即该过滤器的不同行数很低）时，mysql认为，在大多数情况下，准确地说，英尺数会更快。

为了确认，请显示SELECT COUNT(DISTINCT tn.tid) FROM term_node tn的SELECT COUNT(DISTINCT tn.tid) FROM term_node tn SELECT COUNT(*) FROM term_node tn