【发布时间】：2012-04-24 14:31:50
【问题描述】：
我有这个问题：
SELECT
s.last_spread, s.sd, s.mean, s.id
,c.id_ticker, c.coef
,t.ticker
,p.last, p.price
FROM (SELECT * FROM spreads WHERE spreads.id_check=1 LIMIT 100,500 ) as s
INNER JOIN coef as c
ON c.id_spread = s.id
INNER JOIN tickers AS t
ON t.id = c.id_ticker
LEFT JOIN (SELECT prices.id_ticker, MAX(prices.date) as last, prices.price FROM prices GROUP BY prices.id_ticker) AS p
ON p.id_ticker = t.id
这些是表的架构：
mysql> desc spreads;
+-------------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| id_check | int(11) | YES | MUL | NULL | |
| sd | double | YES | | NULL | |
| mean | double | YES | | NULL | |
| last_spread | double | YES | | NULL | |
+-------------+---------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
mysql> desc coef;
+-----------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| id_spread | int(11) | YES | MUL | NULL | |
| id_ticker | int(11) | YES | | NULL | |
| coef | double | YES | | NULL | |
| side | double | YES | | NULL | |
+-----------+---------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
mysql> desc tickers;
+----------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| ticker | varchar(45) | NO | | NULL | |
| name | varchar(150) | NO | | NULL | |
| category | varchar(150) | NO | | NULL | |
| issuer | varchar(150) | NO | | NULL | |
+----------+------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
mysql> desc prices;
+-----------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| id_ticker | int(10) unsigned | NO | MUL | NULL | |
| date | date | NO | | NULL | |
| price | double | NO | | NULL | |
+-----------+------------------+------+-----+---------+----------------+
4 rows in set (0.01 sec)
这些是以上表格的索引；
mysql> show indexes from spreads;
+---------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| spreads | 0 | PRIMARY | 1 | id | A | 2299 | NULL | NULL | | BTREE | |
| spreads | 1 | check_idx | 1 | id_check | A | 1 | NULL | NULL | YES | BTREE | |
+---------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.00 sec)
mysql> show indexes from coef;
+-------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+-------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| coef | 0 | PRIMARY | 1 | id | A | 9078 | NULL | NULL | | BTREE | |
| coef | 1 | spread_ticker_idx | 1 | id_spread | A | NULL | NULL | NULL | YES | BTREE | |
| coef | 1 | spread_ticker_idx | 2 | id_ticker | A | NULL | NULL | NULL | YES | BTREE | |
+-------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
3 rows in set (0.00 sec)
mysql> show indexes from tickers;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| tickers | 0 | PRIMARY | 1 | id | A | 100 | NULL | NULL | | BTREE | |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
1 row in set (0.00 sec)
mysql> show indexes from prices;
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
| prices | 0 | PRIMARY | 1 | id | A | 19962 | NULL | NULL | | BTREE | |
| prices | 1 | id_ticker | 1 | id_ticker | A | 19962 | NULL | NULL | | BTREE | |
+--------+------------+-----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+
2 rows in set (0.15 sec)
这是查询的解释：
+----+-------------+------------+--------+-------------------+-------------------+---------+---------------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+-------------------+---------+---------------------------+--------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 500 | |
| 1 | PRIMARY | c | ref | spread_ticker_idx | spread_ticker_idx | 5 | s.id | 90 | Using where |
| 1 | PRIMARY | t | eq_ref | PRIMARY | PRIMARY | 4 | spreadtrading.c.id_ticker | 1 | Using where |
| 1 | PRIMARY | <derived3> | ALL | NULL | NULL | NULL | NULL | 100 | |
| 3 | DERIVED | prices | index | NULL | id_ticker | 4 | NULL | 119774 | |
| 2 | DERIVED | spreads | ref | check_idx | check_idx | 5 | | 2298 | Using where |
+----+-------------+------------+--------+-------------------+-------------------+---------+---------------------------+--------+-------------+
6 rows in set (0.27 sec)
我可以优化它吗？
谢谢！
编辑：
我想知道索引和表的结构是否针对我上面发布的查询进行了优化。我使用此查询得到的结果很好，效果很好，但也许我可以对其进行优化以提高查询的“速度”。
【问题讨论】：
-
哇。你怎么认为？你试过什么吗？
EXPLAIN输出中的某些表缺少选定的key是否不会引发标志？目前查询需要多长时间？你真的认为你可以把所有这些信息都扔掉并得到答案吗？ -
您对价格的派生查询是最大的罪魁祸首。不要在那里使用派生查询。
-
@eggyal - 好吧，如果 op 刚刚发布了他的查询，那么 我们 会要求他发布表结构和计划，所以我认为这是这些帖子的改进非常受欢迎
-
@Lamak：是的。我不想阻止发布相关信息，而只是呈现它而不试图分析其内容......？
-
@Dail - 我希望你知道，如果没有
ORDER BY在那个spreads子查询中，你可能不会得到你期望的行； SQL 本质上是无序的，并且（尤其是WHERE子句）大多数实现都有优化器来调整语句以高效运行，这可能会修改结果排序。因此，您目前正在获得“随机”行。