如何優化此查詢？

Question

我有兩個MySQL表與後續的結構：

mysql> describe symbol; 
+-------------+--------------+------+-----+---------+----------------+ 
| Field  | Type   | Null | Key | Default | Extra   | 
+-------------+--------------+------+-----+---------+----------------+ 
| id   | int(11)  | NO | PRI | NULL | auto_increment | 
| symbol  | varchar(250) | NO |  | NULL |    | 
+-------------+--------------+------+-----+---------+----------------+ 
2 rows in set (0.00 sec) 

mysql> describe price; 
+-----------+---------+------+-----+---------+----------------+ 
| Field  | Type | Null | Key | Default | Extra   | 
+-----------+---------+------+-----+---------+----------------+ 
| id  | int(11) | NO | PRI | NULL | auto_increment | 
| id_symbol | int(11) | NO | MUL | NULL |    | 
| date  | date | NO | MUL | NULL |    | 
| price  | double | NO |  | NULL |    | 
+-----------+---------+------+-----+---------+----------------+ 
4 rows in set (0.00 sec) 

我要查詢價格表來獲得與兩個符號N個價格清單（對它們進行比較）

這是一個樣品結果：

DATE  SYMBOL 1 SYMBOL 2 
2011-01-01 100  23 
2011-01-02 92  26 
2011-01-03 89  50 

所以我需要找到兩個相同的數據並返回他們的價格。 我已經創建了一個看起來可以完成這項工作的查詢，但可能並不是非常優化。

SELECT * 
FROM price AS a 
    JOIN price AS b ON a.date = b.date 
WHERE a.id_symbol = 1 AND b.id_symbol = 2 
ORDER BY a.date DESC 
LIMIT 100 

您怎麼看？

我可以優化查詢做些什麼（或者可以改變表結構）嗎？

編輯：

這是EXPLAIN {查詢}的

mysql> EXPLAIN SELECT * 
    -> FROM price AS a 
    -> JOIN price AS b ON a.date = b.date 
    -> WHERE a.id_symbol = 1 AND b.id_symbol = 2 
    -> ORDER BY a.date DESC 
    -> LIMIT 100; 
+----+-------------+-------+------+----------------+-----------+---------+-------+------+-----------------------------+ 
| id | select_type | table | type | possible_keys | key  | key_len | ref | rows | Extra      | 
+----+-------------+-------+------+----------------+-----------+---------+-------+------+-----------------------------+ 
| 1 | SIMPLE  | a  | ref | date,id_symbol | id_symbol | 4  | const | 921 | Using where; Using filesort | 
| 1 | SIMPLE  | b  | ref | date,id_symbol | id_symbol | 4  | const | 966 | Using where     | 
+----+-------------+-------+------+----------------+-----------+---------+-------+------+-----------------------------+ 
2 rows in set (0.03 sec) 

你認爲什麼結果呢？

然後......什麼是解釋「行」列中的921和966？如果我數 價格我有符號1和符號2我得到：912和912： - | 是什麼？

（PS是的，我的錯誤，我不得不改變*與我想要的列）

Answer 1

撇開關於過早優化也許是非常合適的意見，你嘗試過使用EXPLAIN（http://dev.mysql.com/doc/refman/5.1/en/using-explain.html）怎麼看你的查詢實際執行？從那裏你可能想看看添加索引。

Answer 2

根據有限的信息，關於您所能做的只是在id_symbol ASC, date DESC上創建一個索引，並可能會將*限制爲僅需要的列。

您最大的「加速」可能是將您的兩個查詢合併爲一個查詢，然後運行該查詢並解析結果集，而不是運行一個查詢來獲取所有符號對，然後查詢另一個查詢每個符號對。您不提供原始查詢，因此無法建議此組合查詢。

Answer 3

這是你的原始查詢

SELECT * 
FROM price AS a 
JOIN price AS b ON a.date = b.date 
WHERE a.id_symbol = 1 AND b.id_symbol = 2 
ORDER BY a.date DESC 
LIMIT 100; 

您需要進行兩個重大變化：

CHANGE＃1：重構您的查詢

只使用密鑰，使何在，ORDER BYS首先發生，JOINs last

SELECT A.date,GROUP_CONCAT(A.price ORDER BY A.id_symbol) prices 
FROM price A 
INNER JOIN 
(
    SELECT BB.* FROM 
    (
    SELECT DISTINCT date FROM 
    (
     SELECT AAAA.date FROM 
     (SELECT date FROM price WHERE id_symbol = 1) AAAA 
     INNER JOIN 
     (SELECT date FROM price WHERE id_symbol = 2) BBBB 
     USING (date) 
     ORDER BY AAAA.date 
    ) AAA ORDER BY date DESC LIMIT 100 
) AA 
    INNER JOIN 
    (SELECT date,id_symbol FROM price WHERE id_symbol in (1,2)) BB 
    USING (date) 
    ORDER BY BB.date DESC,BB.id_symbol 
) B 
USING (date,id_symbol) GROUP BY date; 

CHANGE＃2：CREATE INDEX支持重構QUERY

ALTER TABLE price ADD INDEX symbol_date_id_ndx (id_symbol,date,id); 
ALTER TABLE price ADD INDEX date_id_ndx (date,symbol_id); 

試試看！