为什么我的查询不使用该索引？

Question

I have a query I turned into a view that works OK. But the site_phrase sp table seems to be not using a column and goes through all the records in the table. Why is that? Here is the query:

    EXPLAIN SELECT
             `p`.`id`            AS `id`,
             `p`.`text`          AS `phrase`,
             `p`.`ignored`       AS `ignored_phrase`,
             `p`.`client_id`     AS `client_id`,
             `s`.`id`            AS `site_id`,
             `s`.`sub_domain`    AS `sub_domain`,
             `s`.`competitor`    AS `competitor`,
             `s`.`ignored`       AS `ignored_site`,
             `pc`.`id`           AS `pc_id`,
             `pc`.`name`         AS `pc_name`,
             `psc`.`id`          AS `psc_id`,
             `psc`.`name`        AS `psc_name`,
             `p`.`volume`         AS `volume`,
             MIN(`pos`.`position`) AS `position`,
             `pos`.`id`         AS `pos_id`
  FROM `client` c 
  JOIN client_site cs ON cs.client_id = c.id
  JOIN site s ON s.id = cs.site_id
  JOIN site_phrase sp ON sp.site_id = s.id
  JOIN phrase p ON p.id = sp.phrase_id
  JOIN `position` pos ON pos.phrase_id = sp.phrase_id
    AND pos.site_id = sp.site_id
  LEFT JOIN `phrase_sub_category` `psc`
    ON `psc`.`id` = `p`.`phrase_sub_category_id`
  LEFT JOIN `phrase_category` `pc`
    ON `pc`.`id` = `psc`.`phrase_category_id`
  GROUP BY `p`.`id`,`s`.`id`,`serp`.`id`
  ORDER BY `p`.`id`,`pos`.`position`

And here is a screenshot of the output the above query gets when I EXPLAIN / DESCRIBE it http://img827.imageshack.us/img827/3336/indexsql.png

No matter how I alter the order of the tables above and how they are joined, the 1st or 2nd table always seems to be doing some sort of table scan. In the example of the screenshot, the problem table is sp . These tables are innoDB type, and there is appropriate indexes and foreign keys on all the tables I JOIN on. Any insights would be helpful.

Answer 1

MySQL will use a full table scan if it determines that it is faster than a using index. In the case of your SP table - with only 1300 records the table scan may be just as fast as a index.