alias列会从索引中获益吗?

假设此查询:

SELECT x.*
FROM (
    SELECT COALESCE(p2.id,p1.id) id,
           COALESCE(p2.subject,p2.subject) subject,
           body
    FROM posts p1
    LEFT JOIN posts p2 ON p1.related = p2.id
    WHERE MATCH(subject,body) AGAINST(:entry)
) x INNER JOIN pivot p ON x.id = p.post_id
    INNER JOIN tags  t ON t.id = p.tag_id AND t.name = "tag_name"

x 是一个虚拟表,并且 x.id x、 身份证 pivot 在 pivot.post_id