Mysql Sub Select查詢優化

Question

我每天運行一個查詢來編譯統計信息-但它似乎效率很低。 這是查詢：

SELECT a.id, tstamp, label_id, (SELECT author_id FROM b WHERE b.tid = a.id ORDER BY b.tstamp DESC LIMIT 1) AS author_id
FROM a, b
WHERE (status = '2' OR status = '3') 
AND category != 6
AND a.id = b.tid
AND (b.type = 'C' OR b.type = 'R')
AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.tstamp DESC
LIMIT 500

該查詢似乎運行得很慢。 不好意思的命名-我被要求不要透露實際的表名。

之所以有子選擇，是因為外部選擇從表a中獲得一行，而從表b中獲得一行。 但是還需要知道表b中的最新author_id，因此我運行一個subselect來返回該表。 我不想在php循環內運行另一個選擇-因為這也效率低下。

它可以正常工作-我只需要找到一種更快的方法來獲取此數據集。

Answer 1

嘗試：

  SELECT a.id,
         b.tstamp,
         label_id,
         y.author_id
    FROM TABLE_A a
    JOIN TABLE_B b ON b.tid = a.id
    JOIN (SELECT b.tid,
                 MAX(b.tstamp) 'm_tstamp'
            FROM TABLE_B b
        GROUP BY b.tid) x ON x.tid = a.id
    JOIN (SELECT b.tid,
                 b.author_id,
                 b.tstamp
            FROM TABLE_B b
        GROUP BY b.tid) y ON y.tid = a.id
                         AND y.tstamp = x.m_tstamp
   WHERE status IN ('2', '3')
     AND b.type IN ('C', 'R')
     AND category != 6
     AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
ORDER BY b.tstamp DESC 
   LIMIT 500

Answer 2

如果b.tstamp是內唯一b.tid ，采取OMG小馬的解決方案。

否則，您可以嘗試此解決方案。 它通過b.tstamp DESC對整個結果進行排序，並為每個author_id添加一個排名。 外部選擇僅采用rank = 1的行，這是每個author_id具有最大tstamp author_id 。

SELECT id, tstamp, label_id, author_id
  FROM (SELECT id,
               tstamp,
               label_id,
               author_id,
               CASE
                 WHEN @author_id != author_id THEN @row_num := 1 
                 ELSE @row_num := @row_num + 1
               END AS rank,
               @author_id := b.author_id
          FROM a,
               b,
               (SELECT @row_num := 0, @author_id := NULL) y
          WHERE a.id = b.tid
          AND (status = '2' OR status = '3') 
          AND category != 6
          AND (b.type = 'C' OR b.type = 'R')
          AND a.tstamp1 BETWEEN {$timestamp_start} AND {$timestamp_end}
          ORDER BY b.author_id, b.tstamp DESC
  ) x
 WHERE x.rank = 1
LIMIT 500

我還沒有嘗試過，所以如果它不起作用，請發表評論。