EXPLAIN 命令詳解
阿新 • • 發佈：2018-12-21
在工作中，我們用於捕捉效能問題最常用的就是開啟慢查詢，定位執行效率差的SQL，那麼當我們定位到一個SQL以後還不算完事，我們還需要知道該SQL的執行計劃，比如是全表掃描，還是索引掃描，這些都需要通過EXPLAIN去完成。EXPLAIN命令是檢視優化器如何決定執行查詢的主要方法。可以幫助我們深入瞭解MySQL的基於開銷的優化器，還可以獲得很多可能被優化器考慮到的訪問策略的細節，以及當執行SQL語句時哪種策略預計會被優化器採用。需要注意的是，生成的QEP並不確定，它可能會根據很多因素髮生改變。MySQL不會將一個QEP和某個給定查詢繫結，QEP將由SQL語句每次執行時的實際情況確定，即便使用儲存過程也是如此。儘管在儲存過程中SQL語句都是預先解析過的，但QEP仍然會在每次呼叫儲存過程的時候才被確定。
通過執行計劃可以知道什麼？
([email protected]) [test]>explain select d1.age, t2.id from (select age,name from t1 where id in (1,2))d1, t2 where d1.age=t2.age group by d1.age, t2.id order by t2.id; +----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+ | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 2 | Using temporary; Using filesort | | 1 | PRIMARY | t2 | ref | age | age | 5 | d1.age | 1 | Using where; Using index | | 2 | DERIVED | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where | +----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+ 3 rows in set (0.00 sec) ([email protected]) [test]>
MySQL執行計劃呼叫方式
1.EXPLAIN SELECT ……
變體：
2.EXPLAIN EXTENDED SELECT ……
將執行計劃"反編譯"成SELECT語句，執行SHOW WARNINGS
可得到被MySQL優化器優化後的查詢語句
3.EXPLAIN PARTITIONS SELECT ……
用於分割槽表的EXPLAIN生成QEP的資訊
執行計劃包含的資訊
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1. id：
包含一組數字，表示查詢中執行select子句或操作表的順序
Example（id相同，執行順序由上至下）
([email protected]) [test]>explain select t2.* from t1, t2, t3 where t1.id=t2.id and t1.id=t3.id and t1.name=''; +----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+ | 1 | SIMPLE | t1 | ref | PRIMARY,name | name | 63 | const | 1 | Using where; Using index | | 1 | SIMPLE | t2 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | | | 1 | SIMPLE | t3 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | Using index | +----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+ 3 rows in set (0.00 sec) ([email protected]) [test]>
Example （如果是子查詢，id的序號會遞增，id值越大優先順序越高，越先被執行）
([email protected]) [test]>explain select t2.* from t2 where id = (select id from t1 where id = (select t3.id from t3 where t3.name='')); +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables | | 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table | | 3 | SUBQUERY | t3 | ref | name | name | 63 | | 1 | Using where; Using index | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ 3 rows in set (0.00 sec) ([email protected]) [test]>
Example（id如果相同，可以認為是一組，從上往下順序執行；在所有組中，id值越大，優先順序越高，越先執行）
([email protected]) [test]>explain select t2.* from (select t3.id from t3 where t3.name='')s1, t2 where s1.id=t2.id; +----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+ | 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | | | 1 | PRIMARY | t2 | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 2 | DERIVED | t3 | ref | name | name | 63 | | 1 | Using where; Using index | +----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+ 3 rows in set (0.00 sec) ([email protected]) [test]>
2. select_type
示查詢中每個select子句的型別（簡單OR複雜）
a. SIMPLE：查詢中不包含子查詢或者UNION
b. 查詢中若包含任何複雜的子部分，最外層查詢則被標記為：PRIMARY
c. 在SELECT或WHERE列表中包含了子查詢，該子查詢被標記為：SUBQUERY
d. 在FROM列表中包含的子查詢被標記為：DERIVED（衍生）用來表示包含在from子句中的子查詢的select，mysql會遞迴執行並將結果放到一個臨時表中。伺服器內部稱為"派生表"，因為該臨時表是從子查詢中派生出來的
e. 若第二個SELECT出現在UNION之後，則被標記為UNION；若UNION包含在FROM子句的子查詢中，外層SELECT將被標記為：DERIVED
f. 從UNION表獲取結果的SELECT被標記為：UNION RESULT
SUBQUERY和UNION還可以被標記為DEPENDENT和UNCACHEABLE。
DEPENDENT意味著select依賴於外層查詢中發現的資料。
UNCACHEABLE意味著select中的某些 特性阻止結果被緩存於一個item_cache中。
Example
([email protected]yayun-mysql-server) [test]>explain select d1.name, ( select id from t3) d2 from (select id,name from t1 where name='')d1 union (select name,id from t2); +----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+ | 1 | PRIMARY | <derived3> | system | NULL | NULL | NULL | NULL | 0 | const row not found | | 3 | DERIVED | t1 | ref | name | name | 63 | | 1 | Using where; Using index | | 2 | SUBQUERY | t3 | index | NULL | age | 5 | NULL | 6 | Using index | | 4 | UNION | t2 | index | NULL | name | 63 | NULL | 4 | Using index | | NULL | UNION RESULT | <union1,4> | ALL | NULL | NULL | NULL | NULL | NULL | | +----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+ 5 rows in set (0.00 sec) ([email protected]) [test]>
第一行：id列為1，表示第一個select，select_type列的primary表 示該查詢為外層查詢，table列被標記為<derived3>，表示查詢結果來自一個衍生表，其中3代表該查詢衍生自第三個select查詢，即id為3的select。
第二行：id為3，表示該查詢的執行次序為2（ 4 => 3），是整個查詢中第三個select的一部分。因查詢包含在from中，所以為derived。
第三行：select列表中的子查詢，select_type為subquery，為整個查詢中的第二個select。
第四行：select_type為union，說明第四個select是union裡的第二個select，最先執行。
第五行：代表從union的臨時表中讀取行的階段，table列的<union1,4>表示用第一個和第四個select的結果進行union操作。
3. type
表示MySQL在表中找到所需行的方式，又稱“訪問型別”，常見型別如下:
ALL, index, range, ref, eq_ref, const, system, NULL
從左到右，效能從最差到最好
Example
a. ALL：Full Table Scan， MySQL將遍歷全表以找到匹配的行
([email protected]) [test]>explain select * from t1 where email=''; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
b. index：Full Index Scan，index與ALL區別為index型別只遍歷索引樹
([email protected]) [test]>explain select id from t1; +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | age | 5 | NULL | 4 | Using index | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
c. range:索引範圍掃描，對索引的掃描開始於某一點，返回匹配值域的行。顯而易見的索引範圍掃描是帶有between或者where子句裡帶有<, >查詢。當mysql使用索引去查詢一系列值時，例如IN()和OR列表，也會顯示range（範圍掃描）,當然效能上面是有差異的。
([email protected]) [test]>explain select * from t1 where id in (1,4); +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>explain select * from t1 where id between 1 and 4; +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 3 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>explain select * from t1 where id=1 or id=4; +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.01 sec) ([email protected]) [test]>explain select * from t1 where id > 1; +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 3 | Using where | +----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
d. ref：使用非唯一索引掃描或者唯一索引的字首掃描，返回匹配某個單獨值的記錄行
([email protected]) [test]>explain select * from t1 where name='yayun'; +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+ | 1 | SIMPLE | t1 | ref | name | name | 63 | const | 1 | Using where | +----+-------------+-------+------+---------------+------+---------+-------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
e. eq_ref：類似ref，區別就在使用的索引是唯一索引，對於每個索引鍵值，表中只有一條記錄匹配，簡單來說，就是多表連線中使用primary key或者 unique key作為關聯條件
([email protected]) [test]>explain select t1.name from t1, t2 where t1.id=t2.id; +----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+ | 1 | SIMPLE | t1 | index | PRIMARY | name | 63 | NULL | 4 | Using index | | 1 | SIMPLE | t2 | eq_ref | PRIMARY | PRIMARY | 4 | test.t1.id | 1 | Using index | +----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+ 2 rows in set (0.00 sec) ([email protected]) [test]>
f. const、system：當MySQL對查詢某部分進行優化，並轉換為一個常量時，使用這些型別訪問。如將主鍵置於where列表中，MySQL就能將該查詢轉換為一個常量
([email protected]) [test]>explain select * from ( select * from t1 where id=1)b1; +----+-------------+------------+--------+---------------+---------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+---------------+---------+---------+------+------+-------+ | 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | | | 2 | DERIVED | t1 | const | PRIMARY | PRIMARY | 4 | | 1 | | +----+-------------+------------+--------+---------------+---------+---------+------+------+-------+ 2 rows in set (0.00 sec) ([email protected]) [test]>
注：system是const型別的特例，當查詢的表只有一行的情況下，使用system
g. NULL：MySQL在優化過程中分解語句，執行時甚至不用訪問表或索引，例如從一個索引列裡選取最小值可以通過單獨索引查詢完成。
([email protected]) [test]>explain select * from t1 where id = (select min(id) from t2); +----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+ | 1 | PRIMARY | t1 | const | PRIMARY | PRIMARY | 4 | const | 1 | | | 2 | SUBQUERY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+ 2 rows in set (0.00 sec) ([email protected]) [test]>
4. possible_keys
指出MySQL能使用哪個索引在表中找到記錄，查詢涉及到的欄位上若存在索引，則該索引將被列出，但不一定被查詢使用
5. key
顯示MySQL在查詢中實際使用的索引，若沒有使用索引，顯示為NULL
Example
([email protected]) [test]>explain select id,age from t1; +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | age | 5 | NULL | 4 | Using index | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
6. key_len
表示索引中使用的位元組數，可通過該列計算查詢中使用的索引的長度（key_len顯示的值為索引欄位的最大可能長度，並非實際使用長度，即key_len是根據表定義計算而得，不是通過表內檢索出的）
7. ref
表示上述表的連線匹配條件，即哪些列或常量被用於查詢索引列上的值
8. rows
表示MySQL根據表統計資訊及索引選用情況，估算的找到所需的記錄所需要讀取的行數
Example
([email protected]) [test]>explain select * from t1 , t2 where t1.id=t2.id and t2.name='atlas'; +----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+ | 1 | SIMPLE | t2 | ref | PRIMARY,name | name | 63 | const | 1 | Using where | | 1 | SIMPLE | t1 | eq_ref | PRIMARY | PRIMARY | 4 | test.t2.id | 1 | | +----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+ 2 rows in set (0.00 sec) ([email protected]) [test]>
9. Extra
包含不適合在其他列中顯示但十分重要的額外資訊
a. Using index
該值表示相應的select操作中使用了覆蓋索引（Covering Index）
Example
([email protected]) [test]>explain select id from t1; +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | age | 5 | NULL | 4 | Using index | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
覆蓋索引（Covering Index）
MySQL可以利用索引返回select列表中的欄位，而不必根據索引再次讀取資料檔案
包含所有滿足查詢需要的資料的索引稱為覆蓋索引（Covering Index）
注意：如果要使用覆蓋索引，一定要注意select列表中只取出需要的列，不可select *，因為如果將所有欄位一起做索引會導致索引檔案過大，查詢效能下降
b. Using where
表示mysql伺服器將在儲存引擎檢索行後再進行過濾。許多where條件裡涉及索引中的列，當（並且如果）它讀取索引時，就能被儲存引擎檢驗，因此不是所有帶where字句的查詢都會顯示"Using where"。有時"Using where"的出現就是一個暗示：查詢可受益與不同的索引。
Example
([email protected]) [test]>explain select id,name from t1 where id<4; +----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ | 1 | SIMPLE | t1 | index | PRIMARY | name | 63 | NULL | 4 | Using where; Using index | +----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
c. Using temporary
表示MySQL需要使用臨時表來儲存結果集，常見於排序和分組查詢
這個值表示使用了內部臨時(基於記憶體的)表。一個查詢可能用到多個臨時表。有很多原因都會導致MySQL在執行查詢期間建立臨時表。兩個常見的原因是在來自不同表的上使用了DISTINCT,或者使用了不同的ORDER BY和GROUP BY列。可以強制指定一個臨時表使用基於磁碟的MyISAM儲存引擎。這樣做的原因主要有兩個：
1)內部臨時表佔用的空間超過min(tmp_table_size，max_heap_table_size)系統變數的限制
2)使用了TEXT/BLOB 列
Example
([email protected]ysql-server) [test]>explain select id from t1 where id in (1,2) group by age,name; +----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+ | 1 | SIMPLE | t1 | range | PRIMARY | PRIMARY | 4 | NULL | 2 | Using where; Using temporary; Using filesort | +----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
d. Using filesort
MySQL中無法利用索引完成的排序操作稱為“檔案排序”
Example
([email protected]) [test]>explain select id,age from t1 order by name; +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ | 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 4 | Using filesort | +----+-------------+-------+------+---------------+------+---------+------+------+----------------+ 1 row in set (0.00 sec) ([email protected]) [test]>explain select id,age from t1 order by age; +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t1 | index | NULL | age | 5 | NULL | 4 | Using index | +----+-------------+-------+-------+---------------+------+---------+------+------+-------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
e. Using join buffer
改值強調了在獲取連線條件時沒有使用索引，並且需要連線緩衝區來儲存中間結果。如果出現了這個值，那應該注意，根據查詢的具體情況可能需要新增索引來改進能。
Example
([email protected]) [test]>explain select t1.name from t1 inner join t2 on t1.name=t2.name; +----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+ | 1 | SIMPLE | t1 | index | name | name | 63 | NULL | 4 | Using index | | 1 | SIMPLE | t2 | ref | name | name | 63 | test.t1.name | 2 | Using where; Using index | +----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+ 2 rows in set (0.00 sec) ([email protected]) [test]>alter table t1 drop key name; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 ([email protected]) [test]>alter table t2 drop key name; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 ([email protected]) [test]>explain select t1.name from t1 inner join t2 on t1.name=t2.name; +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ | 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 4 | | | 1 | SIMPLE | t2 | ALL | NULL | NULL | NULL | NULL | 4 | Using where; Using join buffer | +----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+ 2 rows in set (0.00 sec) ([email protected]) [test]>
f. Impossible where
這個值強調了where語句會導致沒有符合條件的行。
Example
([email protected]) [test]>EXPLAIN SELECT * FROM t1 WHERE 1=2; +----+-------------+-------+------+---------------+------+---------+------+------+------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE | +----+-------------+-------+------+---------------+------+---------+------+------+------------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
h. Select tables optimized away
這個值意味著僅通過使用索引，優化器可能僅從聚合函式結果中返回一行.
Example
([email protected]) [test]>explain select max(id) from t1; +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away | +----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+ 1 row in set (0.00 sec) ([email protected]) [test]>
I. Index merges
當MySQL 決定要在一個給定的表上使用超過一個索引的時候，就會出現以下格式中的一個，詳細說明使用的索引以及合併的型別。
Using sort_union(...)
Using union(...)
Using intersect(...)
總結：
• EXPLAIN不會告訴你關於觸發器、儲存過程的資訊或使用者自定義函式對查詢的影響情況
• EXPLAIN不考慮各種Cache
• EXPLAIN不能顯示MySQL在執行查詢時所作的優化工作
• 部分統計資訊是估算的，並非精確值
• EXPALIN只能解釋SELECT操作，其他操作要重寫為SELECT後檢視執行計劃。