MySQL查詢優化

Question

請你幫我優化這個查詢。我花了很多時間，並且仍然無法將其改寫成足夠快的速度（比如說運行在秒鐘之內，而不是現在的分鐘）。

查詢：

SELECT m.my_id, m.my_value, m.my_timestamp 
    FROM (
    SELECT my_id, MAX(my_timestamp) AS most_recent_timestamp 
     FROM my_table 
     WHERE my_timestamp < '2011-03-01 08:00:00' 
     GROUP BY my_id 
) as tmp 
LEFT OUTER JOIN my_table m 
ON tmp.my_id = m.my_id AND tmp.most_recent_timestamp = m.my_timestamp 
ORDER BY m.my_timestamp; 

MY_TABLE定義如下：

CREATE TABLE my_table (
    my_id INTEGER NOT NULL, 
    my_value VARCHAR(4000), 
    my_timestamp TIMESTAMP default CURRENT_TIMESTAMP NOT NULL, 
    INDEX MY_ID_IDX (my_id), 
    INDEX MY_TIMESTAMP_IDX (my_timestamp), 
    INDEX MY_ID_MY_TIMESTAMP_IDX (my_id, my_timestamp) 
); 

該查詢的目標是選擇最近的my_value每個my_id一些時間戳之前。 my_table包含約1億個條目，並且需要大約8分鐘才能完成。

解釋：

 
+----+-------------+-------------+-------+------------------------------------------------+-------------------------+---------+---------------------------+-------+---------------------------------------+ 
| id | select_type | table  | type | possible_keys         | key      | key_len | ref      | rows | Extra         | 
+----+-------------+-------------+-------+------------------------------------------------+-------------------------+---------+---------------------------+-------+---------------------------------------+ 
| 1 | PRIMARY  | <derived2> | ALL | NULL           | NULL     | NULL | NULL      | 90721 | Using temporary; Using filesort  | 
| 1 | PRIMARY  | m   | ref | MY_ID_IDX,MY_TIMESTAMP_IDX,MY_ID_TIMESTAMP_IDX | MY_TIMESTAMP_IDX  | 4  | tmp.most_recent_timestamp | 1 | Using where       | 
| 2 | DERIVED  | my_table | range | MY_TIMESTAMP_IDX        | MY_ID_MY_TIMESTAMP_IDX | 8  | NULL      | 61337 | Using where; Using index for group-by | 
+----+-------------+-------------+-------+------------------------------------------------+-----------------------+---------+---------------------------+------+---------------------------------------+ 

Answer 1

我注意到在解釋計劃中，優化器使用MY_ID_MY_TIMESTAMP_IDX索引爲子查詢，但不是外部查詢。

您可以使用索引提示進行加速。我還更新了ON子句以使用其別名引用tmp.most_recent_timestamp。

SELECT m.my_id, m.my_value, m.my_timestamp 
    FROM (
    SELECT my_id, MAX(my_timestamp) AS most_recent_timestamp 
     FROM my_table 
     WHERE my_timestamp < '2011-03-01 08:00:00' 
     GROUP BY my_id 
) as tmp 
LEFT OUTER JOIN my_table m use index (MY_ID_MY_TIMESTAMP_IDX) 
ON tmp.my_id = m.my_id AND tmp.most_recent_timestamp = m.my_timestamp 
ORDER BY m.my_timestamp; 

Answer 2

一招得到一個最新記錄可以一起與「限1」連同"self" join

財產以後這樣的（未測試）使用order by，而不是max aggregation：

SELECT m.my_id, m.my_value, m.my_timestamp 
FROM my_table m 
WHERE my_timestamp < '2011-03-01 08:00:00' 
ORDER BY m.my_timestamp DESC 
LIMIT 1 
; 

更新以上不會因爲分組工作是必需的...
其他解決方案具有WHERE-IN-SubSelect而不是您使用的JOIN。
可能會更快。請用您的數據進行測試。

SELECT m.my_id, m.my_value, m.my_timestamp 
FROM my_table m 
WHERE (m.my_id, m.my_timestamp) IN (
    SELECT i.my_id, MAX(i.my_timestamp) 
    FROM my_table i 
    WHERE i.my_timestamp < '2011-03-01 08:00:00' 
    GROUP BY i.my_id 
) 
ORDER BY m.my_timestamp; 

Answer 3

如果我理解正確的話，你應該能夠刪除嵌套的選擇完全由my_timestamp下降where子句移動到主查詢，訂單和限制1.

SELECT my_id, my_value, max(my_timestamp) 
FROM my_table 
WHERE my_timestamp < '2011-03-01 08:00:00' 
GROUP BY my_id 

*編輯 - 增加了最大和組