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昨天中午在食堂，和部門的技術(shù)大牛們坐在一桌吃飯，作為一個卑微技術(shù)渣仔默默的吃著飯，聽大佬們高談闊論，研究各種高端技術(shù)，我TM也想說話可實在插不上嘴。

聊著聊著突然說到他上午面試了一個工作6年的程序員，表情挺復(fù)雜，他說：我看他簡歷寫著熟悉SQL語句調(diào)優(yōu)，就問了下 Explain 執(zhí)行計劃怎么看？結(jié)果這老哥一問三不知，工作6年這么基礎(chǔ)的東西都不了解！

感受到了大佬的王之鄙視，回到工位我就開始默默寫這個，哎~ ?我TM也不太懂 Explain ，老哥你這是針對我?。】捱筮髜

Explain有什么用

當(dāng)Explain 與 SQL語句一起使用時，MySQL 會顯示來自優(yōu)化器關(guān)于SQL執(zhí)行的信息。也就是說，MySQL解釋了它將如何處理該語句，包括如何連接表以及什么順序連接表等。

表的加載順序
sql 的查詢類型
可能用到哪些索引，哪些索引又被實際使用
表與表之間的引用關(guān)系
一個表中有多少行被優(yōu)化器查詢 .....

Explain有哪些信息

Explain 執(zhí)行計劃包含字段信息如下：分別是 id、select_type、table、partitions、type、possible_keys、key、key_len、ref、rows、filtered、Extra 12個字段。下邊我們會結(jié)合具體的SQL示例，詳細(xì)的解讀每個字段以及每個字段中不同參數(shù)的含義，以下所有示例數(shù)據(jù)庫版本為 MySQL.5.7.17。

mysql>?select?version()?from?dual;
+------------+
|?version()??|
+------------+
|?5.7.17-log?|
+------------+

我們創(chuàng)建三張表 one、two、three，表之間的關(guān)系 one.two_id = two.two_id AND two.three_id = three.three_id。

Explain執(zhí)行計劃詳解

一、id

id： ：表示查詢中執(zhí)行select子句或者操作表的順序，id的值越大，代表優(yōu)先級越高，越先執(zhí)行。id大致會出現(xiàn) 3種情況：

1、`id`相同

看到三條記錄的id都相同，可以理解成這三個表為一組，具有同樣的優(yōu)先級，執(zhí)行順序由上而下，具體順序由優(yōu)化器決定。

mysql>?EXPLAIN?SELECT?*?FROM?one?o,two?t,?three?r?WHERE?o.two_id?=?t.two_id?AND?t.three_id?=?r.three_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
|?id?|?select_type?|?table?|?partitions?|?type???|?possible_keys?|?key?????|?key_len?|?ref??????????????????|?rows?|?filtered?|?Extra??????????????????????????????????????????????|
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
|??1?|?SIMPLE??????|?o?????|?NULL???????|?ALL????|?NULL??????????|?NULL????|?NULL????|?NULL?????????????????|????2?|??????100?|?NULL???????????????????????????????????????????????|
|??1?|?SIMPLE??????|?t?????|?NULL???????|?ALL????|?PRIMARY???????|?NULL????|?NULL????|?NULL?????????????????|????2?|???????50?|?Using?where;?Using?join?buffer?(Block?Nested?Loop)?|
|??1?|?SIMPLE??????|?r?????|?NULL???????|?eq_ref?|?PRIMARY???????|?PRIMARY?|?4???????|?xin-slave.t.three_id?|????1?|??????100?|?NULL???????????????????????????????????????????????|
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+

2、`id`不同

如果我們的 SQL 中存在子查詢，那么 id的序號會遞增，id值越大優(yōu)先級越高，越先被執(zhí)行。當(dāng)三個表依次嵌套，發(fā)現(xiàn)最里層的子查詢 id最大，最先執(zhí)行。

mysql>?EXPLAIN?select?*?from?one?o?where?o.two_id?=?(select?t.two_id?from?two?t?where?t.three_id?=?(select?r.three_id??from?three?r?where?r.three_name='我是第三表2'));
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|??1?|?PRIMARY?????|?o?????|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????2?|???????50?|?Using?where?|
|??2?|?SUBQUERY????|?t?????|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????2?|???????50?|?Using?where?|
|??3?|?SUBQUERY????|?r?????|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????2?|???????50?|?Using?where?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

3、以上兩種同時存在

將上邊的 SQL 稍微修改一下，增加一個子查詢，發(fā)現(xiàn) id的以上兩種同時存在。相同id劃分為一組，這樣就有三個組，同組的從上往下順序執(zhí)行，不同組 id值越大，優(yōu)先級越高，越先執(zhí)行。

mysql>??EXPLAIN?select?*?from?one?o?where?o.two_id?=?(select?t.two_id?from?two?t?where?t.three_id?=?(select?r.three_id??from?three?r?where?r.three_name='我是第三表2'))?AND?o.one_id?in(select?one_id?from?one?where?o.one_name="我是第一表2");
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type???|?possible_keys?|?key?????|?key_len?|?ref????????????????|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
|??1?|?PRIMARY?????|?o?????|?NULL???????|?ALL????|?PRIMARY???????|?NULL????|?NULL????|?NULL???????????????|????2?|???????50?|?Using?where?|
|??1?|?PRIMARY?????|?one???|?NULL???????|?eq_ref?|?PRIMARY???????|?PRIMARY?|?4???????|?xin-slave.o.one_id?|????1?|??????100?|?Using?index?|
|??2?|?SUBQUERY????|?t?????|?NULL???????|?ALL????|?NULL??????????|?NULL????|?NULL????|?NULL???????????????|????2?|???????50?|?Using?where?|
|??3?|?SUBQUERY????|?r?????|?NULL???????|?ALL????|?NULL??????????|?NULL????|?NULL????|?NULL???????????????|????2?|???????50?|?Using?where?|
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+

二、select_type

select_type：表示 select 查詢的類型，主要是用于區(qū)分各種復(fù)雜的查詢，例如：普通查詢、聯(lián)合查詢、子查詢等。

1、SIMPLE

SIMPLE：表示最簡單的 select 查詢語句，也就是在查詢中不包含子查詢或者 union交并差集等操作。

2、PRIMARY

PRIMARY：當(dāng)查詢語句中包含任何復(fù)雜的子部分，最外層查詢則被標(biāo)記為PRIMARY。

3、SUBQUERY

SUBQUERY：當(dāng) select 或 where 列表中包含了子查詢，該子查詢被標(biāo)記為：SUBQUERY 。

4、DERIVED

DERIVED：表示包含在from子句中的子查詢的select，在我們的 from 列表中包含的子查詢會被標(biāo)記為derived 。

5、UNION

UNION：如果union后邊又出現(xiàn)的select 語句，則會被標(biāo)記為union；若 union 包含在 from 子句的子查詢中，外層 select 將被標(biāo)記為 derived。

6、UNION RESULT

UNION RESULT：代表從union的臨時表中讀取數(shù)據(jù)，而table列的表示用第一個和第四個select的結(jié)果進行union操作。

mysql>?EXPLAIN?select?t.two_name,?(?select?one.one_id?from?one)?o?from?(select?two_id,two_name?from?two?where?two_name?='')?t??union?(select?r.three_name,r.three_id?from?three?r);

+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
|?id???|?select_type??|?table??????|?partitions?|?type??|?possible_keys?|?key?????|?key_len?|?ref??|?rows?|?filtered?|?Extra???????????|
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
|????1?|?PRIMARY??????|?two????????|?NULL???????|?ALL???|?NULL??????????|?NULL????|?NULL????|?NULL?|????2?|???????50?|?Using?where?????|
|????2?|?SUBQUERY?????|?one????????|?NULL???????|?index?|?NULL??????????|?PRIMARY?|?4???????|?NULL?|????2?|??????100?|?Using?index?????|
|????4?|?UNION????????|?r??????????|?NULL???????|?ALL???|?NULL??????????|?NULL????|?NULL????|?NULL?|????2?|??????100?|?NULL????????????|
|?NULL?|?UNION?RESULT?|??|?NULL???????|?ALL???|?NULL??????????|?NULL????|?NULL????|?NULL?|?NULL?|?NULL?????|?Using?temporary?|
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+

三、table

查詢的表名，并不一定是真實存在的表，有別名顯示別名，也可能為臨時表，例如上邊的DERIVED、等。

四、partitions

查詢時匹配到的分區(qū)信息，對于非分區(qū)表值為NULL，當(dāng)查詢的是分區(qū)表時，partitions顯示分區(qū)表命中的分區(qū)情況。

+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table??????????|?partitions??????????????????????|?type??|?possible_keys?|?key?????|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?one????????????|?p201801,p201802,p201803,p300012?|?index?|?NULL??????????|?PRIMARY?|?9???????|?NULL?|????3?|??????100?|?Using?index?|
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+

五、type

type：查詢使用了何種類型，它在 SQL優(yōu)化中是一個非常重要的指標(biāo)，以下性能從好到壞依次是：system ?> const > eq_ref > ref ?> ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

1、system

system：當(dāng)表僅有一行記錄時(系統(tǒng)表)，數(shù)據(jù)量很少，往往不需要進行磁盤IO，速度非?？臁?/span>

2、const

const：表示查詢時命中 primary key 主鍵或者 unique 唯一索引，或者被連接的部分是一個常量(const)值。這類掃描效率極高，返回數(shù)據(jù)量少，速度非?？臁?/span>

mysql>?EXPLAIN?SELECT?*?from?three?where?three_id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|?id?|?select_type?|?table?|?partitions?|?type??|?possible_keys?|?key?????|?key_len?|?ref???|?rows?|?filtered?|?Extra?|
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|??1?|?SIMPLE??????|?three?|?NULL???????|?const?|?PRIMARY???????|?PRIMARY?|?4???????|?const?|????1?|??????100?|?NULL??|
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

3、eq_ref

eq_ref：查詢時命中主鍵primary key 或者 unique key索引， type 就是 eq_ref。

mysql>?EXPLAIN?select?o.one_name?from?one?o?,two?t?where?o.one_id?=?t.two_id?;?
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type???|?possible_keys?|?key??????|?key_len?|?ref????????????????|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
|??1?|?SIMPLE??????|?o?????|?NULL???????|?index??|?PRIMARY???????|?idx_name?|?768?????|?NULL???????????????|????2?|??????100?|?Using?index?|
|??1?|?SIMPLE??????|?t?????|?NULL???????|?eq_ref?|?PRIMARY???????|?PRIMARY??|?4???????|?xin-slave.o.one_id?|????1?|??????100?|?Using?index?|
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+

4、ref

ref：區(qū)別于eq_ref ，ref表示使用非唯一性索引，會找到很多個符合條件的行。

mysql>?select?o.one_id?from?one?o?where?o.one_name?=?"xin"?;?
+--------+
|?one_id?|
+--------+
|??????1?|
|??????3?|
+--------+```

```sql
mysql>?EXPLAIN?select?o.one_id?from?one?o?where?o.one_name?=?"xin"?;?
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??????|?key_len?|?ref???|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|??1?|?SIMPLE??????|?o?????|?NULL???????|?ref??|?idx_name??????|?idx_name?|?768?????|?const?|????1?|??????100?|?Using?index?|
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

5、ref_or_null

ref_or_null：這種連接類型類似于 ref，區(qū)別在于 MySQL會額外搜索包含NULL值的行。

mysql>?EXPLAIN?select?o.one_id?from?one?o?where?o.one_name?=?"xin"?OR?o.one_name?IS?NULL;?
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
|?id?|?select_type?|?table?|?partitions?|?type????????|?possible_keys?|?key??????|?key_len?|?ref???|?rows?|?filtered?|?Extra????????????????????|
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
|??1?|?SIMPLE??????|?o?????|?NULL???????|?ref_or_null?|?idx_name??????|?idx_name?|?768?????|?const?|????3?|??????100?|?Using?where;?Using?index?|
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+

6、index_merge

index_merge：使用了索引合并優(yōu)化方法，查詢使用了兩個以上的索引。

下邊示例中同時使用到主鍵one_id 和字段one_name的idx_name 索引。

mysql>?EXPLAIN?select?*?from?one?o?where?o.one_id?>1?and?o.one_name?='xin';?
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
|?id?|?select_type?|?table?|?partitions?|?type????????|?possible_keys????|?key??????????????|?key_len?|?ref??|?rows?|?filtered?|?Extra??????????????????????????????????????????|
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
|??1?|?SIMPLE??????|?o?????|?NULL???????|?index_merge?|?PRIMARY,idx_name?|?idx_name,PRIMARY?|?772,4???|?NULL?|????1?|??????100?|?Using?intersect(idx_name,PRIMARY);?Using?where?|
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+

7、unique_subquery

unique_subquery：替換下面的 IN子查詢，子查詢返回不重復(fù)的集合。

value?IN?(SELECT?primary_key?FROM?single_table?WHERE?some_expr)

8、index_subquery

index_subquery：區(qū)別于unique_subquery，用于非唯一索引，可以返回重復(fù)值。

value?IN?(SELECT?key_column?FROM?single_table?WHERE?some_expr)

9、range

range：使用索引選擇行，僅檢索給定范圍內(nèi)的行。簡單點說就是針對一個有索引的字段，給定范圍檢索數(shù)據(jù)。在where語句中使用 bettween...and、<、>、<=、in 等條件查詢 type 都是 range。

舉個栗子：three表中three_id為唯一主鍵，user_id普通字段未建索引。

mysql>?EXPLAIN?SELECT?*?from?three?where?three_id?BETWEEN?2?AND?3;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type??|?possible_keys?|?key?????|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?three?|?NULL???????|?range?|?PRIMARY???????|?PRIMARY?|?4???????|?NULL?|????1?|??????100?|?Using?where?|
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

從結(jié)果中看到只有對設(shè)置了索引的字段，做范圍檢索 type 才是 range。

mysql>?EXPLAIN?SELECT?*?from?three?where?user_id?BETWEEN?2?AND?3;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?three?|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|????33.33?|?Using?where?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

10、index

index：Index 與ALL 其實都是讀全表，區(qū)別在于index是遍歷索引樹讀取，而ALL是從硬盤中讀取。

下邊示例：three_id 為主鍵，不帶 where 條件全表查詢，type結(jié)果為index 。

mysql>?EXPLAIN?SELECT?three_id?from?three?;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type??|?possible_keys?|?key?????|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?three?|?NULL???????|?index?|?NULL??????????|?PRIMARY?|?4???????|?NULL?|????1?|??????100?|?Using?index?|
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

11、ALL

ALL：將遍歷全表以找到匹配的行，性能最差。

mysql>?EXPLAIN?SELECT?*?from?two?;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|??1?|?SIMPLE??????|?two???|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????2?|??????100?|?NULL??|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

六、possible_keys

possible_keys：表示在MySQL中通過哪些索引，能讓我們在表中找到想要的記錄，一旦查詢涉及到的某個字段上存在索引，則索引將被列出，但這個索引并不定一會是最終查詢數(shù)據(jù)時所被用到的索引。具體請參考上邊的例子。

七、key

key：區(qū)別于possible_keys，key是查詢中實際使用到的索引，若沒有使用索引，顯示為NULL。具體請參考上邊的例子。

當(dāng) type 為 index_merge 時，可能會顯示多個索引。

八、key_len

key_len：表示查詢用到的索引長度（字節(jié)數(shù)），原則上長度越短越好。

單列索引，那么需要將整個索引長度算進去；
多列索引，不是所有列都能用到，需要計算查詢中實際用到的列。

注意：key_len只計算where條件中用到的索引長度，而排序和分組即便是用到了索引，也不會計算到key_len中。

九、ref

ref：常見的有：const，func，null，字段名。

當(dāng)使用常量等值查詢，顯示const，
當(dāng)關(guān)聯(lián)查詢時，會顯示相應(yīng)關(guān)聯(lián)表的關(guān)聯(lián)字段
如果查詢條件使用了表達式、函數(shù)，或者條件列發(fā)生內(nèi)部隱式轉(zhuǎn)換，可能顯示為func
其他情況null

十、rows

rows：以表的統(tǒng)計信息和索引使用情況，估算要找到我們所需的記錄，需要讀取的行數(shù)。

這是評估SQL 性能的一個比較重要的數(shù)據(jù)，mysql需要掃描的行數(shù)，很直觀的顯示 SQL 性能的好壞，一般情況下 rows 值越小越好。

mysql>?EXPLAIN?SELECT?*?from?three;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|??1?|?SIMPLE??????|?three?|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|??????100?|?NULL??|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

十一、filtered

filtered 這個是一個百分比的值，表里符合條件的記錄數(shù)的百分比。簡單點說，這個字段表示存儲引擎返回的數(shù)據(jù)在經(jīng)過過濾后，剩下滿足條件的記錄數(shù)量的比例。

在MySQL.5.7版本以前想要顯示filtered需要使用explain extended命令。MySQL.5.7后，默認(rèn)explain直接顯示partitions和filtered的信息。

十二、Extra

Extra ：不適合在其他列中顯示的信息，Explain 中的很多額外的信息會在 Extra 字段顯示。

1、Using index

Using index：我們在相應(yīng)的 select 操作中使用了覆蓋索引，通俗一點講就是查詢的列被索引覆蓋，使用到覆蓋索引查詢速度會非?？?，SQl優(yōu)化中理想的狀態(tài)。

什么又是覆蓋索引?

一條 SQL只需要通過索引就可以返回，我們所需要查詢的數(shù)據(jù)（一個或幾個字段），而不必通過二級索引，查到主鍵之后再通過主鍵查詢整行數(shù)據(jù)（select * ）。

one_id表為主鍵

mysql>?EXPLAIN?SELECT?one_id?from?one?;
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type??|?possible_keys?|?key????????|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?one???|?NULL???????|?index?|?NULL??????????|?idx_two_id?|?5???????|?NULL?|????3?|??????100?|?Using?index?|
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+

注意：想要使用到覆蓋索引，我們在 select 時只取出需要的字段，不可select *，而且該字段建了索引。

mysql>?EXPLAIN?SELECT?*?from?one?;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|??1?|?SIMPLE??????|?one???|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|??????100?|?NULL??|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

2、Using where

Using where：查詢時未找到可用的索引，進而通過where條件過濾獲取所需數(shù)據(jù)，但要注意的是并不是所有帶where語句的查詢都會顯示Using where。

下邊示例create_time 并未用到索引，type 為 ALL，即MySQL通過全表掃描后再按where條件篩選數(shù)據(jù)。

mysql>?EXPLAIN?SELECT?one_name?from?one?where?create_time?='2020-05-18';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?one???|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|????33.33?|?Using?where?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

3、Using temporary

Using temporary：表示查詢后結(jié)果需要使用臨時表來存儲，一般在排序或者分組查詢時用到。

mysql>?EXPLAIN?SELECT?one_name?from?one?where?one_id?in?(1,2)?group?by?one_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?one???|?NULL???????|?range|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|????33.33?|?Using?where;?Using?temporary;?Using?filesort?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

4、Using filesort

Using filesort：表示無法利用索引完成的排序操作，也就是ORDER BY的字段沒有索引，通常這樣的SQL都是需要優(yōu)化的。

mysql>?EXPLAIN?SELECT?one_id?from?one??ORDER?BY?create_time;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra??????????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|??1?|?SIMPLE??????|?one???|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|??????100?|?Using?filesort?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

如果ORDER BY字段有索引就會用到覆蓋索引，相比執(zhí)行速度快很多。

mysql>?EXPLAIN?SELECT?one_id?from?one??ORDER?BY?one_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|?id?|?select_type?|?table?|?partitions?|?type??|?possible_keys?|?key?????|?key_len?|?ref??|?rows?|?filtered?|?Extra???????|
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|??1?|?SIMPLE??????|?one???|?NULL???????|?index?|?NULL??????????|?PRIMARY?|?4???????|?NULL?|????3?|??????100?|?Using?index?|
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

5、Using join buffer

Using join buffer：在我們聯(lián)表查詢的時候，如果表的連接條件沒有用到索引，需要有一個連接緩沖區(qū)來存儲中間結(jié)果。

先看一下有索引的情況：連接條件 one_name 、two_name 都用到索引。

mysql>?EXPLAIN?SELECT?one_name?from?one?o,two?t?where?o.one_name?=?t.two_name;
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
|?id?|?select_type?|?table?|?partitions?|?type??|?possible_keys?|?key??????|?key_len?|?ref??????????????????|?rows?|?filtered?|?Extra????????????????????|
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
|??1?|?SIMPLE??????|?o?????|?NULL???????|?index?|?idx_name??????|?idx_name?|?768?????|?NULL?????????????????|????3?|??????100?|?Using?where;?Using?index?|
|??1?|?SIMPLE??????|?t?????|?NULL???????|?ref???|?idx_name??????|?idx_name?|?768?????|?xin-slave.o.one_name?|????1?|??????100?|?Using?index??????????????|
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+

接下來刪掉連接條件 one_name 、two_name 的字段索引。發(fā)現(xiàn)Extra 列變成 Using join buffer，type均為全表掃描，這也是SQL優(yōu)化中需要注意的地方。

mysql>?EXPLAIN?SELECT?one_name?from?one?o,two?t?where?o.one_name?=?t.two_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra??????????????????????????????????????????????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
|??1?|?SIMPLE??????|?t?????|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????2?|??????100?|?NULL???????????????????????????????????????????????|
|??1?|?SIMPLE??????|?o?????|?NULL???????|?ALL??|?NULL??????????|?NULL?|?NULL????|?NULL?|????3?|????33.33?|?Using?where;?Using?join?buffer?(Block?Nested?Loop)?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+

6、Impossible where

Impossible where：表示在我們用不太正確的where語句，導(dǎo)致沒有符合條件的行。

mysql>?EXPLAIN?SELECT?one_name?from?one?WHERE?1=2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra????????????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
|??1?|?SIMPLE??????|?NULL??|?NULL???????|?NULL?|?NULL??????????|?NULL?|?NULL????|?NULL?|?NULL?|?NULL?????|?Impossible?WHERE?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+

7、No tables used

No tables used：我們的查詢語句中沒有FROM子句，或者有 FROM DUAL子句。

mysql>?EXPLAIN?select?now();
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|?id?|?select_type?|?table?|?partitions?|?type?|?possible_keys?|?key??|?key_len?|?ref??|?rows?|?filtered?|?Extra??????????|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|??1?|?SIMPLE??????|?NULL??|?NULL???????|?NULL?|?NULL??????????|?NULL?|?NULL????|?NULL?|?NULL?|?NULL?????|?No?tables?used?|
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

Extra列的信息非常非常多，這里就不再一一列舉了，詳見 MySQL官方文檔：https://dev.mysql.com/doc/refman/5.7/en/explain-output.html#jointype_index_merge

總結(jié)

上邊只是簡單介紹了下 Explain 執(zhí)行計劃各個列的含義，了解它不僅僅是要應(yīng)付面試，在實際開發(fā)中也經(jīng)常會用到。比如對慢SQL進行分析，如果連執(zhí)行計劃結(jié)果都不會看，那還談什么SQL優(yōu)化呢？

面試官:不會看 Explain執(zhí)行計劃,簡歷敢寫 SQL 優(yōu)化?

Explain有什么用

Explain有哪些信息

Explain執(zhí)行計劃詳解

一、id

1、id相同

2、id不同

3、以上兩種同時存在

二、select_type

1、SIMPLE

2、PRIMARY

3、SUBQUERY

4、DERIVED

5、UNION

6、UNION RESULT

三、table

四、partitions

五、type

1、system

2、const

3、eq_ref

4、ref