title: Mysql query is too slow when order by datetime
body: <p>I have a table with about 10k rows. I run below sql but it's take too much time (~4 seconds):</p>
  <pre><code>SELECT
          ID, post_date, post_title, post_name, pc.post_view,
          t.name AS post_category,
          pm2.meta_value AS post_image
      FROM wp_posts p
          INNER JOIN wp_term_relationships tr ON p.ID = tr.object_id
          INNER JOIN wp_term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id AND tt.taxonomy='category'
          INNER JOIN wp_terms t ON tt.term_id = t.term_id AND (t.name='My Posts' OR t.name='HOT posts')
          LEFT JOIN post_counter pc ON p.ID = pc.post_id
          LEFT JOIN wp_postmeta pm ON pm.post_id = p.ID AND pm.meta_key = '_thumbnail_id'
          LEFT JOIN wp_postmeta pm2 ON pm.meta_value = pm2.post_id AND pm2.meta_key = '_wp_attached_file'
      WHERE p.post_status = 'publish' AND p.post_type = 'post'
      ORDER BY FIELD(t.name, 'HOT posts', 'My Posts'), post_date DESC
      LIMIT 14
  </code></pre>
  <p>If I remove &quot;order by post_date&quot;, everything fine. It's take only 0.05 second to run.</p>
  <p>I added some indexes with CREATE INDEX and ADD INDEX (I don't know what's difference between them):</p>
  <pre><code>CREATE INDEX ix_type_date ON wp_posts (post_type, post_date)
  
  ALTER TABLE wp_posts ADD INDEX ix_status_type_date (post_status,post_type,post_date)
  </code></pre>
  <p>Then use these indexes with USE INDEX and FORCE INDEX (I also don't know what's difference between them):</p>
  <pre><code>    ...FROM wp_posts p USE INDEX (ix_type_date)...
  </code></pre>
  <p>OR</p>
  <pre><code>    ...FROM wp_posts p FORCE INDEX (ix_status_type_date)...
  </code></pre>
  <p>I still get my records, but loading time increased to 7-8 seconds.</p>
  <p>Table's structure</p>
  <pre><code>ID  bigint(20)  No           
  post_author bigint(20)  No  0        
  post_date   datetime    No  0000-00-00 00:00:00          
  post_date_gmt   datetime    No  0000-00-00 00:00:00          
  post_content    longtext    No           
  post_title  text    No           
  post_excerpt    text    No           
  post_status varchar(20) No  publish          
  comment_status  varchar(20) No  open         
  ping_status varchar(20) No  open         
  post_password   varchar(255)    No           
  post_name   varchar(200)    No           
  to_ping text    No           
  pinged  text    No           
  post_modified   datetime    No  0000-00-00 00:00:00          
  post_modified_gmt   datetime    No  0000-00-00 00:00:00          
  post_content_filtered   longtext    No           
  post_parent bigint(20)  No  0        
  guid    varchar(255)    No           
  menu_order  int(11) No  0        
  post_type   varchar(20) No  post         
  post_mime_type  varchar(100)    No           
  comment_count   bigint(20)  No  0 
  </code></pre>