一个非常基本的表，包含利用TokuDB存储引擎的多个交换机上的工具的引号：
CREATE TABLE `quotes` (
`ticker` char(4) NOT NULL,
`timestamp` time(3) NOT NULL,
`price` decimal(7,2) unsigned NOT NULL,
`size` smallint(5) unsigned NOT NULL,
`exchange` char(3) NOT NULL,
KEY `best_price` (`ticker`,`timestamp`,`exchange`,`price`),
KEY `best_size` (`exchange`,`ticker`,`price`,`timestamp`)
) ENGINE=TokuDB
每当我在所有交易所查询最优价格时，它总是使用临时表。索引中存在exchange和price只会产生索引扫描，等同于TokuDB中(ticker, timestamp)上的聚簇键。
EXPLAIN SELECT max(price),exchange
FROM quotes
WHERE
ticker="A" AND
timestamp BETWEEN "15:15:22.328961" AND "15:17:22.328961"
GROUP BY exchange
ORDER BY NULL \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: quotes
type: range
possible_keys: best_price
key: best_price
key_len: 9
ref: NULL
rows: 2690
Extra: Using where; Using index; Using temporary
是否可以定义不使用临时表的配置？只有在删除timestamp文章时才会出现这种情况：
EXPLAIN SELECT max(price),exchange
FROM quotes
WHERE
ticker="A"
GROUP BY exchange
ORDER BY NULL \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: quotes
type: range
possible_keys: best_price
key: best_size
key_len: 7
ref: NULL
rows: 96
Extra: Using where; Using index for group-by
最佳价格查询的示例输出：
+------------+----------+
| max(price) | exchange |
+------------+----------+
| 41.06 | BTY |
| 41.06 | DEA |
| 41.07 | NYS |
| 41.07 | THM |
| 41.06 | PSE |
| 41.07 | BAT |
| 41.06 | DEX |
| 41.06 | BOS |
| 41.06 | ADC |
| 41.06 | XPH |
+------------+----------+
10 rows in set (0.01 sec)
瓶颈（3ms）正在处理时间范围内的每一行：
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000071 |
| checking permissions | 0.000005 |
| Opening tables | 0.000016 |
| After opening tables | 0.000006 |
| System lock | 0.000014 |
| Table lock | 0.000002 |
| After table lock | 0.000005 |
| init | 0.000038 |
| optimizing | 0.000024 |
| statistics | 0.000155 |
| preparing | 0.000028 |
| executing | 0.000003 |
| Copying to tmp table | 0.000031 |
| Copying to tmp table | 0.003381 |
| Sending data | 0.000017 |
| end | 0.000004 |
| removing tmp table | 0.000020 |
| end | 0.000002 |
| query end | 0.000005 |
| closing tables | 0.000005 |
| freeing items | 0.000006 |
| updating status | 0.000011 |
| cleaning up | 0.000002 |
+----------------------+----------+
时间范围包含2316行，按交换细分：
+----------+----------+
| exchange | count(*) |
+----------+----------+
| ADC | 71 |
| BAT | 298 |
| BOS | 129 |
| BTY | 266 |
| DEA | 153 |
| DEX | 60 |
| NYS | 530 |
| PSE | 325 |
| THM | 453 |
| XPH | 31 |
+----------+----------+
我已经尝试了 crazy 并添加了覆盖索引的每个排列，并且MariaDB无法找到更好的密钥。是否还有其他我应该关注的数据库？
时间范围和股票代码的示例数据集：http://pastebin.com/b5RcTXAs
答案 0 :(得分：0)
答案就是优化。 MySQL选择使用临时表，因为它被认为比使用索引获取每个交换更合理。如果在exchange,ticker,timestamp之间使用聚簇索引，则查询可以在没有临时的情况下运行：
MariaDB [trth]> explain SELECT min(ask_price),exchange
FROM quotes
USE INDEX (exchange_ticker_timestamp)
WHERE exchange IN ("NYS","BOS","CIN","ADC","DEX","DEA","MID","PSE","THM","WCB","BAT","XPH","BTY") AND
ticker="A" AND
timestamp BETWEEN "15:15:22.328961" AND "15:17:22.328961"
GROUP BY exchange
ORDER BY NULL \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: quotes
type: range
possible_keys: exchange_ticker_timestamp
key: exchange_ticker_timestamp
key_len: 10
ref: NULL
rows: 2589
Extra: Using where; Using index
MariaDB [trth]> show profile;
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000079 |
| checking permissions | 0.000006 |
| Opening tables | 0.000014 |
| After opening tables | 0.000011 |
| System lock | 0.000014 |
| Table lock | 0.000003 |
| After table lock | 0.000005 |
| init | 0.000043 |
| optimizing | 0.000019 |
| statistics | 0.000234 |
| preparing | 0.000027 |
| executing | 0.000008 |
| Sorting result | 0.000002 |
| Sending data | 0.002985 |
| end | 0.000006 |
| query end | 0.000010 |
| closing tables | 0.000006 |
| freeing items | 0.000007 |
| updating status | 0.000138 |
| cleaning up | 0.000004 |
+----------------------+----------+
与临时表分组比较：
MariaDB [trth]> explain SELECT min(ask_price),exchange
FROM quotes
WHERE ticker="A" AND
timestamp BETWEEN "15:15:22.328961" AND "15:17:22.328961"
GROUP BY exchange
ORDER BY NULL \G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: quotes
type: range
possible_keys: ticker_timestamp
key: ticker_timestamp
key_len: 9
ref: NULL
rows: 1515
Extra: Using where; Using temporary
MariaDB [trth]> show profile;
+----------------------+----------+
| Status | Duration |
+----------------------+----------+
| starting | 0.000091 |
| checking permissions | 0.000009 |
| Opening tables | 0.000037 |
| After opening tables | 0.000009 |
| System lock | 0.000052 |
| Table lock | 0.000004 |
| After table lock | 0.000009 |
| init | 0.000049 |
| optimizing | 0.000025 |
| statistics | 0.000144 |
| preparing | 0.000039 |
| executing | 0.000003 |
| Copying to tmp table | 0.000040 |
| Copying to tmp table | 0.004674 |
| Sending data | 0.000020 |
| end | 0.000003 |
| removing tmp table | 0.000015 |
| end | 0.000003 |
| query end | 0.000004 |
| closing tables | 0.000006 |
| freeing items | 0.000006 |
| updating status | 0.000204 |
| cleaning up | 0.000004 |
+----------------------+----------+
这里有趣的是，第一个查询扫描更多行但执行速度比第二个更快。