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mysql table combine_Mysql系列-性能优化神器EXPLAIN使用介绍及分析

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MySQL 提供了一個 EXPLAIN 命令, 它可以對?SELECT?語句進行分析, 并輸出?SELECT?執行的詳細信息, 以供開發人員針對性優化。

EXPLAIN 命令用法十分簡單, 在 SELECT 語句前加上 Explain 就可以了, 例如:

EXPLAIN SELECT * from user_info WHERE id < 300;

下面是我結合我自己創建的表以及執行相關sql語句總結的相關知識點。

準備

為了接下來方便演示 EXPLAIN 的使用, 首先我們需要建立兩個測試用的表, 并添加相應的數據:

DROP TABLE IF EXISTS`customers`;CREATE TABLE`customers` (

`customerNumber`int(11) NOT NULL,

`customerName`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`contactLastName`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`contactFirstName`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`phone`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`addressLine1`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`addressLine2`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,

`city`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`state`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,

`postalCode`varchar(15) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL,

`country`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`salesRepEmployeeNumber`int(11) NULL DEFAULT NULL,

`creditLimit`decimal(10, 2) NULL DEFAULT NULL,PRIMARY KEY(`customerNumber`) USING BTREE,INDEX`salesRepEmployeeNumber`(`salesRepEmployeeNumber`) USING BTREE,INDEX`customers_idx_combine_1`(`customerName`, `phone`, `customerNumber`) USING BTREE,CONSTRAINT `customers_ibfk_1` FOREIGN KEY (`salesRepEmployeeNumber`) REFERENCES `employees` (`employeeNumber`) ON DELETE RESTRICT ON UPDATE RESTRICT) ENGINE= InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

DROP TABLE IF EXISTS`employees`;CREATE TABLE`employees` (

`employeeNumber`int(11) NOT NULL,

`lastName`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`firstName`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`extension`varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`email`varchar(100) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`officeCode`varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,

`reportsTo`int(11) NULL DEFAULT NULL,

`jobTitle`varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,PRIMARY KEY(`employeeNumber`) USING BTREE,INDEX`reportsTo`(`reportsTo`) USING BTREE,INDEX`officeCode`(`officeCode`) USING BTREE

) ENGINE= InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;

創建表成功后，插入一些測試數據。

EXPLAIN 輸出格式

EXPLAIN 命令的輸出內容大致如下:

mysql> EXPLAIN SELECT * FROM customers WHERE customerName='Herkku Gifts' AND phone='+47 2267 3215' AND customerNumber=167;+----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+

+----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+

+----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

各列的含義如下:

id: SELECT 查詢的標識符. 每個 SELECT 都會自動分配一個唯一的標識符

select_type: SELECT 查詢的類型

table: 查詢的是哪個表

partitions: 匹配的分區

type: join 類型

possible_keys: 此次查詢中可能選用的索引

key: 此次查詢中確切使用到的索引

ken_len:?表示查詢優化器使用了索引的字節數

ref: 哪個字段或常數與 key 一起被使用

rows: 顯示此查詢一共掃描了多少行. 這個是一個估計值.

filtered: 表示此查詢條件所過濾的數據的百分比

extra: 額外的信息

接下來我們詳細看一下每個字段的具體含義：

select_type

select_type?表示了查詢的類型, 它的常用取值有:

SIMPLE：表示此查詢不包含 UNION 查詢或子查詢

PRIMARY：表示此查詢是最外層的查詢

UNION：表示此查詢是 UNION 的第二或隨后的查詢

DEPENDENT UNION：UNION 中的第二個或后面的查詢語句, 取決于外面的查詢

UNION RESULT：UNION 的結果

SUBQUERY：子查詢中的第一個 SELECT

DEPENDENT SUBQUERY：子查詢中的第一個 SELECT, 取決于外面的查詢. 即子查詢依賴于外層查詢的結果

DERIVED：當子查詢是from子句時，其select_type為DERIVED

最常見的查詢類別應該是?SIMPLE了, 比如當我們的查詢沒有子查詢, 也沒有 UNION 查詢時, 那么通常就是?SIMPLE類型, 例如：

1.SIMPLE?情況：

+----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+

+----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

2.UNION情況

當通過union來連接多個查詢結果時，第二個之后的select其select_type為UNION

mysql> EXPLAIN SELECT customerNumber FROM customers WHERE customerNumber IN (125,144) UNION SELECT customerNumber FROM customers WHERE country IN ('USA','France');+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+

+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+

+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+

3 rows in set, 1 warning (0.00 sec)

3.DEPENDENT UNION與DEPENDENT SUBQUERY

當union作為子查詢時，其中第二個union的select_type就是DEPENDENT UNION。第一個子查詢的select_type則是DEPENDENT SUBQUERY

mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber IN (SELECT customerNumber FROM customers WHERE customerNumber IN (125,144) UNION SELECT customerNumber FROM customers WHERE country IN ('USA','France'));+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+

+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+

+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+

4 rows in set, 1 warning (0.00 sec)

4.SUBQUERY

子查詢中的第一個select其select_type為SUBQUERY

mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber=(SELECT customerNumber FROM customers WHERE customerNumber=124);+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

2 rows in set, 1 warning (0.00sec)

mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber in (SELECT customerNumber FROM customers WHERE customerNumber=124);+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

2 rows in set, 1 warning (0.00 sec)

思考下為什么一個用了in一個用了=反而select_type就不一樣了？？？？

5.DERIVED

mysql> EXPLAIN SELECT * FROM (SELECT COUNT(*) FROM customers WHERE customerNumber=124) a;+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+

2 rows in set, 1 warning (0.00 sec)

table

表示查詢涉及的表或衍生表

partitions

匹配的分區

type

type字段比較重要, 顯示連接使用了何種類型。從最好到最差的連接類型依次分別為const、eq_reg、ref、range、index和ALL它提供了判斷查詢是否高效的重要依據依據。

通過?type字段, 我們判斷此次查詢是?全表掃描?還是?索引掃描?等。

type顯示的是訪問類型，是較為重要的一個指標，結果值從好到壞依次是：system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

一般來說，得保證查詢至少達到range級別，最好能達到ref。

type常用的取值有:

system: 表中只有一條數據. 這個類型是特殊的?const類型。

const: 針對主鍵或唯一索引的等值查詢掃描, 最多只返回一行數據。const 查詢速度非常快, 因為它僅僅讀取一次即可。

表示通過索引一次就找到了，const用于比較primary key 或者 unique索引。因為只需匹配一行數據，所有很快。如果將主鍵置于where列表中，mysql就能將該查詢轉換為一個const 。

例如下面的這個查詢, 它使用了主鍵索引, 因此?type就是?const類型的

mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber=128;+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

eq_ref：唯一性索引掃描，對于每個索引鍵，表中只有一條記錄與之匹配。常見于主鍵或唯一索引掃描。

mysql> SELECT * FROMitems;+----+---------+

| id | item_no |

+----+---------+

| 1 | A-1 |

| 2 | A-2 |

| 3 | A-3 |

| 4 | A-4 |

| 5 | A-5 |

| 6 | A-10 |

| 7 | A-11 |

| 8 | A-20 |

| 9 | A-30 |

+----+---------+

9 rows in set (0.00 sec)

mysql> SELECT * FROM customers LIMIT 10;+----------------+------------------------------+-----------------+------------------+-------------------+------------------------------+--------------+---------------+----------+------------+-----------+------------------------+-------------+

+----------------+------------------------------+-----------------+------------------+-------------------+------------------------------+--------------+---------------+----------+------------+-----------+------------------------+-------------+

| 112 | Signal Gift Stores | King | Jean | 7025551838 | 8489 Strong St. | NULL | Las Vegas | NV | 83030 | USA | 1166 | 71800.00 |

| 124 | Mini Gifts Distributors Ltd. | Nelson | Susan | 4155551450 | 5677 Strong St. | NULL | San Rafael | CA | 97562 | USA | 1165 | 210500.00 |

| 129 | Mini Wheels Co. | Murphy | Julie | 6505555787 | 5557 North Pendale Street | NULL | San Francisco | CA | 94217 | USA | 1165 | 64600.00 |

| 131 | Land of Toys Inc. | Lee | Kwai | 2125557818 | 897 Long Airport Avenue | NULL | NYC | NY | 10022 | USA | 1323 | 114900.00 |

10 rows in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM items,customers WHERE customers.customerNumber=items.id;+----+-------------+-----------+------------+--------+---------------+---------+---------+-------------------+------+----------+-------+

+----+-------------+-----------+------------+--------+---------------+---------+---------+-------------------+------+----------+-------+

+----+-------------+-----------+------------+--------+---------------+---------+---------+-------------------+------+----------+-------+

2 rows in set, 1 warning (0.00 sec)

注意：ALL全表掃描的表是記錄最少的表如items表。

ref：非唯一性索引掃描，返回匹配某個單獨值的所有行。本質是也是一種索引訪問，它返回所有匹配某個單獨值的行，然而他可能會找到多個符合條件的行，所以它應該屬于查找和掃描的混合體。

mysql> SELECT * FROM customers WHERE salesRepEmployeeNumber=1165;+----------------+------------------------------+-----------------+------------------+------------+---------------------------+--------------+---------------+-------+------------+---------+------------------------+-------------+

+----------------+------------------------------+-----------------+------------------+------------+---------------------------+--------------+---------------+-------+------------+---------+------------------------+-------------+

| 124 | Mini Gifts Distributors Ltd. | Nelson | Susan | 4155551450 | 5677 Strong St. | NULL | San Rafael | CA | 97562 | USA | 1165 | 210500.00 |

| 129 | Mini Wheels Co. | Murphy | Julie | 6505555787 | 5557 North Pendale Street | NULL | San Francisco | CA | 94217 | USA | 1165 | 64600.00 |

| 161 | Technics Stores Inc. | Hashimoto | Juri | 6505556809 | 9408 Furth Circle | NULL | Burlingame | CA | 94217 | USA | 1165 | 84600.00 |

| 321 | Corporate Gift Ideas Co. | Brown | Julie | 6505551386 | 7734 Strong St. | NULL | San Francisco | CA | 94217 | USA | 1165 | 105000.00 |

| 450 | The Sharp Gifts Warehouse | Frick | Sue | 4085553659 | 3086 Ingle Ln. | NULL | San Jose | CA | 94217 | USA | 1165 | 77600.00 |

| 487 | Signal Collectibles Ltd. | Taylor | Sue | 4155554312 | 2793 Furth Circle | NULL | Brisbane | CA | 94217 | USA | 1165 | 60300.00 |

6 rows in set (0.00sec)

mysql> EXPLAIN SELECT * FROM customers WHERE salesRepEmployeeNumber=1165;+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+

+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+

| 1 | SIMPLE | customers | NULL | ref | salesRepEmployeeNumber| salesRepEmployeeNumber | 5 | const | 6 | 100.00 | NULL |

+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

mysql> show index fromcustomers;+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| customers | 1 | salesRepEmployeeNumber | 1 | salesRepEmployeeNumber| A | 16 | NULL | NULL | YES | BTREE | | |

+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

5 rows in set (0.00 sec)

range：只檢索給定范圍的行，使用一個索引來選擇行。key列顯示使用了那個索引。一般就是在where語句中出現了bettween、、in等的查詢。這種索引列上的范圍掃描比全索引掃描要好。只需要開始于某個點，結束于另一個點，不用掃描全部索引。

mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber BETWEEN 1 AND 120;+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

index：Full Index Scan，index與ALL區別為index類型只遍歷索引樹。這通常為ALL快，因為索引文件通常比數據文件小。(Index與ALL雖然都是讀全表，但index是從索引中讀取，而ALL是從硬盤讀取) 。

mysql> EXPLAIN SELECT id FROMitems;+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00sec)

mysql> show index fromitems;+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| items | 0 | PRIMARY | 1 | id | A | 9 | NULL | NULL | | BTREE | | |

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

1 row in set (0.00 sec)

ALL：Full Table Scan，遍歷全表以找到匹配的行。

mysql> EXPLAIN SELECT item_no FROMitems;+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

1 row in set, 1 warning (0.00sec)

mysql> show index fromitems;+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| items | 0 | PRIMARY | 1 | id | A | 9 | NULL | NULL | | BTREE | | |

+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

1 row in set (0.00 sec)

possible_keys

查詢涉及到的字段上存在索引，則該索引將被列出，但不一定被查詢實際使用。

key

實際使用的索引，如果為NULL，則沒有使用索引。

查詢中如果使用了覆蓋索引，則該索引僅出現在key列表中。

+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

| customers | 1 | salesRepEmployeeNumber | 1 | salesRepEmployeeNumber | A | 16 | NULL | NULL | YES | BTREE | | |

+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

5 rows in set (0.00sec)

mysql> EXPLAIN SELECT customerName,phone,customerNumber fromcustomers;+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

上述customerName,phone,customerNumber與customers_idx_combine_1索引順序一致，為覆蓋索引

mysql> EXPLAIN SELECT phone,customerName,customerNumber fromcustomers;+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN SELECT customerName,phone,customerNumber,state fromcustomers;+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------+

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------+

+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------+

1 row in set, 1 warning (0.00 sec)

state字段為非索引列。

key_len

表示索引中使用的字節數，查詢中使用的索引的長度(最大可能長度)，并非實際使用長度，理論上長度越短越好。key_len是根據表定義計算而得的，不是通過表內檢索出的。

ref

顯示索引的那一列被使用了，如果可能，是一個常量const。

mysql> EXPLAIN SELECT customerNumber FROM customers WHERE customerNumber=114;+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+

1 row in set, 1 warning (0.00 sec)

rows

根據表統計信息及索引選用情況，大致估算出找到所需的記錄所需要讀取的行數。

Extra

不適合在其他字段中顯示，但是十分重要的額外信息。

Using filesort?:mysql對數據使用一個外部的索引排序，而不是按照表內的索引進行排序讀取。也就是說mysql無法利用索引完成的排序操作成為“文件排序” 。

總結

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