WordPress使用NOT EXISTS的meta_query引发慢查询问题求助

我的数据库规模不大：wp_posts表有10000行，wp_postmeta表有100000行，但某条WP_Query查询执行耗时约5-7秒。

传入的WP_Query参数 #

Array(
   [post_type] => post
   [post_status] => publish
   [ignore_sticky_posts] => 0
   [posts_per_page] => 1
   [orderby] => rand
   [meta_key] => 
   [meta_value] => 
   [order] => DESC
   [offset] => 0
   [post__not_in] => Array
       (
       )

   [author__in] => Array
       (
       )

   [tax_query] => Array
       (
           [relation] => AND
           [0] => Array
               (
                   [taxonomy] => category
                   [field] => slug
                   [terms] => Array
                       (
                           [0] => beauty
                       )

                   [operator] => IN
                   [include_children] => 0
               )

       )

   [meta_query] => Array
       (
           [relation] => OR
           [0] => Array
               (
                   [key] => sponsored
                   [value] => 1
                   [compare] => !=
                   [type] => CHAR
               )

           [1] => Array
               (
                   [key] => sponsored
                   [value] => 
                   [compare] => NOT EXISTS
                   [type] => CHAR
               )

       )

)

生成的SQL查询语句 #

SELECT SQL_CALC_FOUND_ROWS wp_posts.id
FROM   wp_posts
       LEFT JOIN wp_term_relationships
              ON ( wp_posts.id = wp_term_relationships.object_id )
       LEFT JOIN wp_postmeta
              ON ( wp_posts.id = wp_postmeta.post_id )
       LEFT JOIN wp_postmeta AS mt1
              ON ( wp_posts.id = mt1.post_id
                   AND mt1.meta_key = 'sponsored' )
WHERE  1 = 1
       AND ( wp_term_relationships.term_taxonomy_id IN ( 3 ) )
       AND ( ( wp_postmeta.meta_key = 'sponsored'
               AND wp_postmeta.meta_value != '1' )
              OR mt1.post_id IS NULL )
       AND wp_posts.post_type = 'post'
       AND (( wp_posts.post_status = 'publish' ))
GROUP  BY wp_posts.id
ORDER  BY Rand()
LIMIT  0, 1

问题定位 #

我发现移除以下meta_query条件后，慢查询问题消失：

Array(
   [key] => sponsored
   [value] => 
   [compare] => NOT EXISTS
   [type] => CHAR
)

对应的SQL语句部分为：

OR mt1.post_id IS NULL

需求 #

我需要筛选出标记为sponsored == 0或未设置sponsored元键的文章，恳请帮忙提供更高效的WP_Query/SQL查询方案，或者解释该条件导致查询耗时激增的原因。

内容的提问来源于stack exchange，提问作者PavKR