012--Explain實戰(zhàn)詳解：Extra分析

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Extra分析：包含不在其他屬性顯示雳窟，但是又非常重要的信息

1.Using FileSort：說明MySQL會對數(shù)據(jù)使用一個外部的索引排序，而不是按照表內的索引順序進行讀取，即MySQL無法使用索引完成的排序稱為"文件排序"

2.Using temporary：使用了臨時表來保存中間結果，MYSQL在對查詢結果進行排序的時候使用了臨時表，常見于排序OrderBy 和分組查詢GroupBy

Using FileSort只是不能按照索引方法進行排序淤击，但是Using temporary會創(chuàng)建一張臨時表，將緩存數(shù)據(jù)存放在臨時表中故源，然后再刪除臨時表污抬，操作變得更兇險了

3.Using Index：

表示相應的select操作中使用了覆蓋索引（Covering Index），避免訪問了表的數(shù)據(jù)行绳军，效率不錯
如果同時出現(xiàn)Using Where印机，表明索引被用來執(zhí)行索引鍵值的查找
如果沒有同時出現(xiàn)Using Where，表明索引用來讀取數(shù)據(jù)而非執(zhí)行查找工作

4.覆蓋索引：

select 查詢的數(shù)據(jù)列只用ongoing索引中就能夠取得门驾，不必讀取數(shù)據(jù)行射赛，MySQL可以利用索引返回select列表中的字段，而不必根據(jù)索引再次讀取數(shù)據(jù)文件奶是，換句話說：索引列要被所建的索引覆蓋

如果想使用覆蓋索引楣责，在select的時候不能使用select * from t，具體寫出對應的字段

5.Using Where：使用where過濾條件

6.Using Join Buffer：使用了連接緩存（join太多個表聂沙，配置文件里面的JoinBuffer的值可以調大一點）

7.Impossible Where：where子句的值總是false秆麸，不能獲取任何元組

8.Select tables optimized away：在沒有GroupBy子句的情況下，基于索引優(yōu)化Min/Max操作或者對于MyISAM存儲引擎優(yōu)化Count（*）操作及汉，不必等到執(zhí)行階段再進行計算沮趣，查詢執(zhí)行計劃生成的階段即完成優(yōu)化

9.distinct：優(yōu)化distinct操作，在找到第一個匹配的元組后即停止找同樣值的動作

1.主鍵自動創(chuàng)建唯一索引
2.頻繁作為查詢的條件的字段應該創(chuàng)建索引
3.查詢中與其他表關聯(lián)的字段坷随，外鍵關系建立索引
4.頻繁更新的字段不應該創(chuàng)建索引房铭，因為每次更新不單單是更新了記錄，還會更新索引温眉，非常耗時
5.where條件中用不到的字段不適合創(chuàng)建索引
6.單鍵育叁、組合索引的選擇問題，（在高并發(fā)下傾向于創(chuàng)建組合索引）
7.查詢中排序的字段芍殖，排序字段若通過索引去訪問將大大的提高排序速度
8.查選中統(tǒng)計或分組的字段

索引和排序是有關系的：建完索引之后，排序也要按照索引建立的順序進行排序谴蔑，要不然會造成索引失效現(xiàn)象

1.1（A）沒有創(chuàng)建關聯(lián)索引

mysql>  explain SELECT * from t4 ORDER BY t4.id;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+
|  1 | SIMPLE      | t4    | index | NULL          | PRIMARY | 4       | NULL |    8 |       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.id,t4.`name`;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t4    | ALL  | NULL          | NULL | NULL    | NULL |    8 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.id,t4.`name`,t4.age;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t4    | ALL  | NULL          | NULL | NULL    | NULL |    8 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

1.1（B）關聯(lián)索引

mysql> explain SELECT * from t4 ORDER BY t4.id;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+
|  1 | SIMPLE      | t4    | index | NULL          | PRIMARY | 4       | NULL |    8 |       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.id,t4.`name`;
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key               | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-------------+
|  1 | SIMPLE      | t4    | index | NULL          | index_id_name_age | 42      | NULL |    8 | Using index |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-------------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.id,t4.`name`,t4.age;
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key               | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-------------+
|  1 | SIMPLE      | t4    | index | NULL          | index_id_name_age | 42      | NULL |    8 | Using index |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-------------+
1 row in set (0.00 sec)

?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?

1.2（A）沒有創(chuàng)建關聯(lián)索引

mysql> explain SELECT * from t4 ORDER BY t4.`name`,t4.id;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t4    | ALL  | NULL          | NULL | NULL    | NULL |    8 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.`name`,t4.id,t4.age;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t4    | ALL  | NULL          | NULL | NULL    | NULL |    8 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.`name`,t4.age,t4.id;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t4    | ALL  | NULL          | NULL | NULL    | NULL |    8 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

1.2（B）創(chuàng)建關聯(lián)索引

mysql> explain SELECT * from t4 ORDER BY t4.`name`,t4.id;
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
| id | select_type | table | type  | possible_keys | key               | key_len | ref  | rows | Extra                       |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | t4    | index | NULL          | index_id_name_age | 42      | NULL |    8 | Using index; Using filesort |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.`name`,t4.id,t4.age;
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
| id | select_type | table | type  | possible_keys | key               | key_len | ref  | rows | Extra                       |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | t4    | index | NULL          | index_id_name_age | 42      | NULL |    8 | Using index; Using filesort |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4 ORDER BY t4.`name`,t4.age,t4.id;
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
| id | select_type | table | type  | possible_keys | key               | key_len | ref  | rows | Extra                       |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | t4    | index | NULL          | index_id_name_age | 42      | NULL |    8 | Using index; Using filesort |
+----+-------------+-------+-------+---------------+-------------------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)

?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?

1.3（A）沒有創(chuàng)建關聯(lián)索引

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.id;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.id,t4.`name`;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.id,t4.`name`,t4.age;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

1.3（B）創(chuàng)建關聯(lián)索引

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.id;
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys             | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY,index_id_name_age | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.id,t4.`name`;
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys             | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY,index_id_name_age | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.id,t4.`name`,t4.age;
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys             | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY,index_id_name_age | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?

1.4（A）沒有創(chuàng)建關聯(lián)索引

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.`name`,t4.id;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.`name`,t4.id,t4.age;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.`name`,t4.age,t4.id;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

1.4（B）創(chuàng)建關聯(lián)索引

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.`name`,t4.id;
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys             | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY,index_id_name_age | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.`name`,t4.id,t4.age;
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys             | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY,index_id_name_age | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

mysql> explain SELECT * from t4  where t4.id = '1' ORDER BY t4.`name`,t4.age,t4.id;
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys             | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t4    | const | PRIMARY,index_id_name_age | PRIMARY | 4       | const |    1 |       |
+----+-------------+-------+-------+---------------------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)

2.創(chuàng)建索引代碼

CREATE INDEX index_id_name_age ON t4 (id,`name`,age)

?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?　?

最后編輯于：2017.12.10 04:23:10

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012--Explain實戰(zhàn)詳解：Extra分析

1.Using FileSort：說明MySQL會對數(shù)據(jù)使用一個外部的索引排序，而不是按照表內的索引順序進行讀取，即MySQL無法使用索引完成的排序稱為"文件排序"

2.Using temporary：使用了臨時表來保存中間結果，MYSQL在對查詢結果進行排序的時候使用了臨時表，常見于排序OrderBy 和分組查詢GroupBy

3.Using Index：

表示相應的select操作中使用了覆蓋索引（Covering Index），避免訪問了表的數(shù)據(jù)行绳军，效率不錯

如果同時出現(xiàn)Using Where印机，表明索引被用來執(zhí)行索引鍵值的查找

如果沒有同時出現(xiàn)Using Where，表明索引用來讀取數(shù)據(jù)而非執(zhí)行查找工作

4.覆蓋索引：

5.Using Where：使用where過濾條件

6.Using Join Buffer：使用了連接緩存（join太多個表聂沙，配置文件里面的JoinBuffer的值可以調大一點）

7.Impossible Where：where子句的值總是false秆麸，不能獲取任何元組

8.Select tables optimized away：在沒有GroupBy子句的情況下，基于索引優(yōu)化Min/Max操作或者對于MyISAM存儲引擎優(yōu)化Count（*）操作及汉，不必等到執(zhí)行階段再進行計算沮趣，查詢執(zhí)行計劃生成的階段即完成優(yōu)化

9.distinct：優(yōu)化distinct操作，在找到第一個匹配的元組后即停止找同樣值的動作

1.1（A）沒有創(chuàng)建關聯(lián)索引

1.1（B）關聯(lián)索引

1.2（A）沒有創(chuàng)建關聯(lián)索引

1.2（B）創(chuàng)建關聯(lián)索引

1.3（A）沒有創(chuàng)建關聯(lián)索引

1.3（B）創(chuàng)建關聯(lián)索引

1.4（A）沒有創(chuàng)建關聯(lián)索引

1.4（B）創(chuàng)建關聯(lián)索引

2.創(chuàng)建索引代碼

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