Mysql之索引及優化
性能分析
MySql Query Optimizer
自身select語句優化模塊（未必是DBA認爲最優的）
mysql常見瓶頸
CPU：CPU在飽和的時候通常發生在數據裝入內存或從磁盤上讀取數據的時候
IO：磁盤IO瓶頸發生在裝入數據遠大於內存容量的時候
服務器硬件性能瓶頸
Explain
模擬優化器執行sql語句，從而知道mysql是如何處理sql語句的，從而分析出性能瓶頸node
MyISAM 不支持外鍵 不支持事務 表鎖 只緩存索引，不緩存數據 表空間小 關注性能
InnoDB 支持外鍵 支持事務 行鎖 緩存索引及數據 表空間大 關注事務mysql
Percona的xtradb引擎是一款能夠替代InnoDB的引擎，在性能和併發上作的更好算法
多表查詢sql
select <select_list> from tableA A left join tableB B on A.key=B.key 右補NULL select <select_list> from tableA A left join tableB B on A.key=B.key where B.key=NULL A獨有 select <select_list> from tableA A right join tableB B on A.key=B.key 左補NULL select <select_list> from tableA A right join tableB B on A.key=B.key where A.key=NULL B獨有 select <select_list> from tableA A inner join tableB B on A.key=B.key 共有 select <select_list> from tableA A full outer join tableB B on A.key=B.key 全有（mysql不支持，變通爲左聯unin右聯） select <select_list> from tableA A full outer join tableB B on A.key=B.key where A.key=NULL or where B.key=NULL A獨加B獨（mysql不支持，變通爲左獨聯unin右獨聯）
什麼是索引
是幫助Mysql高效獲取數據的數據結構，簡單理解爲「排好序的快速查找數據結構」,即索引兩大功能，排序和查找
數據自己以外，數據庫還維護這一個知足特定查找算法的數據結構，這些數據結構以某種方式指向數據，這樣就能夠在這些數據結構的基礎上實現高級查找算法，這種數據結構就是索引
一般使用BTREE索引（多路搜索樹） ，其餘索引結構有hash。full-text,r-tree數據庫
優勢
提升數據檢索效率，下降數據庫的IO成本
經過索引列對數據進行排序，下降數據排序成本，下降CPU消耗緩存
缺點
索引也是一張表，保存主鍵與索引字段，並指向實體記錄，因此因此列也是要佔用空間
提升了查詢速度，可是會下降更新，即增刪改的速度
索引只是提升效率的一個因素，若是有大數據量的表，須要花時間創建最優索引服務器
delete從邏輯刪除，將記錄標記爲inactive，大量的inactive記錄影響數據查找，穩定後有必要重建索引session
建立索引基本知識
一、索引相似書的目錄，會加快查詢速度
二、在表的列（字段）上建立索引
三、索引會加快查詢速度，可是也會影響更新速度，由於更新會維護索引數據。update修改數據時，會自動更新索引，因此有時會慢
四、索引不是越多越好，通常選擇查詢頻繁的where條件的字段上建立
五、小表或重複值不少的列上通常不建索引，要在大表及重複值少的條件列上建立索引
六、多個列聯合索引有前綴生效特性
七、當字段內容前n個字符已經接近惟一時，能夠對字段前n個字符建立索引
八、索引從工做方式分，有主鍵，惟一，普通索引
九、索引類型有BTREE（默認）和hash（適合緩存，內存數據庫）等數據結構
索引列的建立及生效條件
問題1?既然索引能夠加快查詢速度，那麼就給全部列建索引吧？
解答：索引不但佔用系統空間，更新數據庫時還須要維護索引數據，所以索引是一把雙刃劍，不是越多越好。幾十到幾百行的小表無需創建索引，寫多讀少的業務少建索引。
問題2？到底在哪些列上創建索引呢
secect user,host from mysql.user where host=...索引必定要建立在查詢頻繁條件列，而不是selcect後的選擇數據的列，另外咱們要儘可能選擇在惟一值多的大表上的列創建索引。select count(distinct列) from table併發
適合建立索引的場景
主鍵自動創建惟一索引
頻繁做爲查詢條件的字段應該建立索引
查詢中與其餘表關聯的字段，外鍵關係創建索引
頻繁更新的字段不適合建索引
where條件裏用不到的字段不建索引
單鍵/組合索引的選擇：在高併發下傾向建立組合索引
查詢中排序的字段，排序字段若經過索引去訪問將大大提升排序速度。即創建了inx_col1_col2_col3，order by col,col3優於order by col1
查詢中統計或者分組字段
不適合建立索引的場景
表記錄較少 （300w內）
常常增刪改的表
數據分佈平均的表字段或者重複多。例若有1000條記錄，有999個惟一，索引選擇性是999/1000.越接近於1，效率就越高
索引的創建與刪除
=>增長索引在訪問低谷時進行，幾百萬條須要幾分鐘
=>uniq index惟一索引，（用來約束字段值惟一，好比網站註冊時用的email惟一，查詢速度會快些）
mysql> show index from student; +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | student | 0 | PRIMARY | 1 | id | A | 4 | NULL | NULL | | BTREE | | | | student | 1 | idx_name | 1 | names | A | 4 | NULL | NULL | YES | BTREE | | | +---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ mysql> drop index idx_name on student; mysql> create index idx_name on student(names); mysql> alter table student drop index idx_name; mysql> alter table student add index idx_name(names);
索引分類
單值
惟一
複合
alter table tb_name add primary key(col_list) 主鍵索引，索引值必須惟一，不能爲NUll
alter table tb_name add unique index_name(col_list) 建立的索引值必須惟一（除了NULL，NULL可能會出現屢次）
alter table tb_name add index_name(col_list） 普通索引，索引值可出現屢次
alter table tb_name add fulltext index_name(col_list） 全文索引
對字段的前n個字符建立索引
mysql> select * from student; +----+--------+-------------+-----+--------+----------+ | id | names | phonenum | age | dept | qq | +----+--------+-------------+-----+--------+----------+ | 1 | 張三 | 13701800003 | 23 | 經理 | 9485754 | | 2 | 李四 | 13701800004 | 24 | 助理 | 12344754 | | 3 | 王五 | 13701800005 | 25 | 員工 | 5414754 | | 4 | 趙六 | 1370180006 | 26 | 員工 | 1209756 | +----+--------+-------------+-----+--------+----------+
mysql> create index idx_qq4 on student(qq(4));
mysql> show index from student\G
*************************** 3. row ***************************
Table: student
Non_unique: 1
Key_name: idx_qq4
Seq_in_index: 1
Column_name: qq
Collation: A
Cardinality: 4
Sub_part: 4
Packed: NULL
Null: YES
Index_type: BTREE
Comment:
Index_comment:
建立聯合索引
/*按照列查詢數據時，聯合索引是有前綴生效特性的
/*index（a,b,c）僅僅a,ab,abc三個查詢條件列能夠走索引，其餘不走索引，因此把常常用於條件查詢的放前面
mysql> create index idx_names_qq on student(names,qq(4)); mysql> show index from student\G *************************** 4. row *************************** Table: student Non_unique: 1 Key_name: idx_names_qq Seq_in_index: 1 Column_name: names Collation: A Cardinality: 4 Sub_part: NULL Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment: *************************** 5. row *************************** Table: student Non_unique: 1 Key_name: idx_names_qq Seq_in_index: 2 Column_name: qq Collation: A Cardinality: 4 Sub_part: 4 Packed: NULL Null: YES Index_type: BTREE Comment: Index_comment:
MYSQL查詢優化 explain做用(sql語句執行計劃)
- 表的讀取順序 id
- 數據讀取操做的操做類型
- 哪些索引可使用 possible_keys
- 哪些索引被實際使用 key
- 表之間的引用 ref
- 每張表有多少行被優化器查詢 rows
mysql> explain select * from student where qq="5414754"; +----+-------------+---------+------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | student | ref | idx_qq4 | idx_qq4 | 13 | const | 1 | Using where | +----+-------------+---------+------+---------------+---------+---------+-------+------+-------------+
id:select
查詢的序列號，包含一組數字，表示查詢中執行select子句或操做表的順序 1、id相同，執行順序由上至下 2、id不一樣，若是是子查詢，id的序號會遞增，越大優先級越高，越先被執行 3、id有相同有不一樣，大的先執行，相同的順序執行 select_type:
主要是用於區別普通查詢、聯合查詢、子查詢或是 SIMPLE 簡單的select查詢，查詢中部包含子查詢或是UNION PRIMARY 查詢中若包含任何複雜查詢的子部分，最外層查詢被標記爲PRIMARY SUBQUERY 在SELECT或WHERE列表中包含了子查詢 DERIVED 在FROM列表中包含的子查詢被標記爲DERIVED（衍生），MYsql會遞歸執行這些子查詢，把結果放在臨時表 UNION 若第二個SELECT出如今UNION後，則標記爲UNION，若UNION包含在FROM子句的子查詢中，外層SELECT將被標記爲DERIVED UNION RESULT 從UNION表獲取結果的SELECT type:
訪問類型排列，顯示了查詢使用了何種類型，經常使用的好到差依次爲system>const>eq_ref>ref>range>index>ALL，#通常來講，得保證查詢至少達到range級別，最好的達到ref system 表只有一行記錄（等於系統表，這是const類型的特列，這個能夠忽略不計） const 表示經過索引一次就查找到，const常見於primary key或者unique索引，由於只有匹配一行數據，因此很快，若是主鍵置於where列表，Mysql就能將該查詢轉換爲一個常量 eq_ref 惟一性索引掃描，對於每一個索引鍵，表中只有一條記錄與之匹配，常見於主鍵或惟一性索引掃描 ref 非惟一性索引掃描，返回匹配某個單獨值得全部行，本質上也是一種索引訪問，它返回全部匹配某個單獨值的行，然而它可能會找到多個符合條件的行，因此他應該屬於查找和掃描的混合體 range 只檢索給定範圍的行，使用一個索引來選擇行，key列顯示使用了哪一個索引，通常就是在where語句中出現了between、<、>、in等條件，這種範圍掃描索引比全表掃描要好，由於它只須要開始於索引的某點，而結束於另外一個點，不用掃描所有索引 index Full Index scan，（全索引掃描）index和ALL的區別爲index類型只遍歷索引樹，一般比ALL快，由於索引文件一般比數據文件小。也就是說雖然all和index都是讀全表，可是index是從索引中讀取，而all則是從硬盤中讀取 all Full Table scan，將遍歷全表以找到匹配的行,全表掃描 possible_keys
顯示可能應用在這張表中的索引，一個或多個，查詢涉及到的字段上若存在索引，則該索引被列出，可是不必定被使用
key
實際使用的索引，若是爲NULL，則沒有使用索引。查詢中若使用到了覆蓋索引（聯合索引個數與select項順序及個數吻合），則該索引僅出如今key列表中
key_len
表示索引中使用的字節數，可經過該列計算查詢中使用的索引長度，在不損失精度的狀況下，長度越短越好，key_len顯示的值爲索引字段的最大可能長度，並不是實際使用長度。即key_len是根據表定義計算而得，不是經過表內檢索出的
ref
顯示索引的哪一列被使用了，若是可能的話，最好是一個常量，哪些列或常量被用於查找索引列上的值
rows
根據表統計信息及索引選用狀況，大體估算出找到所需的記錄所須要讀取的行數 extra
包含在不適合在其餘列中顯示但十分重要的額外信息 Using filesort（差） 說明mysql會對數據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，mysql中沒法利用索引完成的排序操做稱爲「文件排序」 Using temporary（更差） 使用了臨時表保存中間結果，Mysql在對查詢結果排序時使用了臨時表，常見於排序order by和分組查詢group by Using index（好） 表示相應的select操做中使用到了覆蓋索引（Covering Index），避免訪問了表的數據行，效率不錯 若是同時出現using where，代表索引被用來執行索引鍵值的查找 若是沒有同時出現using where，代表索引被用來讀取數據而非執行查找動做 Covering Index （覆蓋索引）：就是select的數據列只用從索引中就能取得，沒必要讀取數據行，mysql能夠利用索引返回select表中的字段，而沒必要根據索引再次讀取數據文件，換句話說，查詢列要被所創建的索引覆蓋。
若是要使用覆蓋索引，必定要注意select取出須要的列，不可select * Using where Using join buffer（不重要，少見） 使用了鏈接緩存 impossible where （不重要，少見） where子句的值老是false，不能用來獲取任何元組（where name=「a」 and name=「b」） select tables optimized away（不重要，少見） 在沒有group by子句的狀況下，基於索引優化min/max操做或者對於myisam存儲引擎優化count（*）操做，沒必要等到執行階段再進行計算，查詢執行計劃生成的階段即完成優化 distinct（不重要，少見） 優化distinct操做，在找到第一匹配的元組後即中止找一樣值得動做
mysql> drop index idx_qq4 on student; mysql> explain select * from student where qq="5414754"; +----+-------------+---------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+---------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | student | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+---------+------+---------------+------+---------+------+------+-------------+
索引分析
單表
兩表
三表
單表
#建立測試數據
create table article ( id int(10) unsigned not null primary key auto_increment, auther_id int(10) unsigned not null, category_id int(10) unsigned not null, view int(10) unsigned not null, comments int(10) unsigned not null, title varbinary(255) not null, content text not null ); insert into article values (1,1,1,1,1,"1","1"); insert into article values (2,2,2,2,2,"2","1"); insert into article values (3,3,3,3,3,"3","3");
#查詢category_id=1，且comments>1,view最多的auther_id
mysql> explain select id,auther_id from article where category_id=1 and comments>1 order by view desc limit 1;
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------------------+
| 1 | SIMPLE | article | ALL | NULL | NULL | NULL | NULL | 3 | Using where; Using filesort |
+----+-------------+---------+------+---------------+------+---------+------+------+-----------------------------+
mysql> create index idx_article_ccv on article(category_id,comments,view);
mysql> explain select id,auther_id from article where category_id=1 and comments>1 order by view desc limit 1; #comments>1範圍之後索引致使失效
+----+-------------+---------+-------+-----------------+-----------------+---------+------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+-------+-----------------+-----------------+---------+------+------+-----------------------------+
| 1 | SIMPLE | article | range | idx_article_ccv | idx_article_ccv | 8 | NULL | 1 | Using where; Using filesort |
+----+-------------+---------+-------+-----------------+-----------------+---------+------+------+-----------------------------+
==>BTREE索引工做原理，A，B，C聯合索引，先排序A，若是有相同的，再排序B，B有相同的，排序C。上述案例由於comments>1條件是個範圍，所以，沒法再利用索引對其後字段進行檢索
mysql> explain select id,auther_id from article where category_id=1 and comments=1 order by view desc limit 1;
+----+-------------+---------+------+-----------------+-----------------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+-----------------+-----------------+---------+-------------+------+-------------+
| 1 | SIMPLE | article | ref | idx_article_ccv | idx_article_ccv | 8 | const,const | 1 | Using where |
+----+-------------+---------+------+-----------------+-----------------+---------+-------------+------+-------------+
mysql> drop index idx_article_ccv on article;
#第一次優化不理想，嘗試第二次
mysql> create index idx_article_cv on article(category_id,view);
mysql> explain select id,auther_id from article where category_id=1 and comments>1 order by view desc limit 1;
+----+-------------+---------+------+----------------+----------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------+------+----------------+----------------+---------+-------+------+-------------+
| 1 | SIMPLE | article | ref | idx_article_cv | idx_article_cv | 4 | const | 1 | Using where |
+----+-------------+---------+------+----------------+----------------+---------+-------+------+-------------+
兩表
create table class ( id int(10) unsigned not null auto_increment, card int(10) unsigned not null, primary key (id) ); create table book ( bookid int(10) unsigned not null auto_increment, card int(10) unsigned not null, primary key (bookid) ); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into class(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20))); insert into book(card) values (FLOOR(1+(RAND()*20)));
mysql> explain select * from class left join book on class.card=book.card;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | class | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | SIMPLE | book | ALL | NULL | NULL | NULL | NULL | 7 | |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
mysql> alter table book add index Y(card);
mysql> explain select * from class left join book on class.card=book.card; #左鏈接，右索引的狀況
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
| 1 | SIMPLE | class | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | SIMPLE | book | ref | Y | Y | 4 | gtms.class.card | 1 | Using index |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
mysql> drop index Y on book
mysql> alter table class add index Y(card);
mysql> explain select * from class left join book on class.card=book.card; #左鏈接，左索引的狀況
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | class | index | NULL | Y | 4 | NULL | 10 | Using index |
| 1 | SIMPLE | book | ALL | NULL | NULL | NULL | NULL | 7 | |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
==>結論，leftjoin條件用於肯定如何從右表搜索行，左邊必定都有，因此右邊是咱們的關鍵點，必定須要創建索引。
==>即左鏈接，使用右索引。class left join book=book right join class,交換表位置，因此索引不變，可是能夠改變這個
==>右鏈接同理
三表
建立測試數據 create table phone ( phoneid int(10) unsigned not null auto_increment, card int(10) unsigned not null, primary key (phoneid) ) engine=innodb; insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20))); insert into phone(card) values (FLOOR(1+(RAND()*20)));
mysql> explain select * from class left join book on class.card=book.card left join phone on book.card=phone.card;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | class | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | SIMPLE | book | ALL | NULL | NULL | NULL | NULL | 7 | |
| 1 | SIMPLE | phone | ALL | NULL | NULL | NULL | NULL | 12 | |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
在右2表加索引
mysql> alter table phone add index Z(card);
mysql> alter table book add index Y(card);
mysql> explain select * from class left join book on class.card=book.card left join phone on book.card=phone.card;
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
| 1 | SIMPLE | class | ALL | NULL | NULL | NULL | NULL | 10 | |
| 1 | SIMPLE | book | ref | Y | Y | 4 | gtms.class.card | 1 | Using index |
| 1 | SIMPLE | phone | ref | Z | Z | 4 | gtms.book.card | 1 | Using index |
+----+-------------+-------+------+---------------+------+---------+-----------------+------+-------------+
==>後兩行type都是ref且總row優化效果很好，所以索引最好設置在須要常常查詢的字段上
結論jion語句優化
儘量減小join語句中的nestedloop的循環總次數，「永遠用小結果集驅動大的結果集」 即，左鏈接，左邊所有，右邊部分，右邊小，所以索引建在右
優先優化nestedloop的內層循環
保證jion語句中被驅動表上的join條件字段已經被索引
當沒法保證被驅動表的join條件字段被索引且內存資源充足的前提下，不要太吝嗇JoinBuffer的設置
避免索引失效注意點
全值匹配我最愛 最佳左前綴法則 不在索引列上作任何操做（計算、函數、類型轉換（自動或手動）），會致使索引失效而轉向全表掃描 存儲引擎不能使用索引中範圍條件右邊的列 儘可能使用覆蓋索引，減小select * mysql在使用不等於（!=或<>）的時候沒法使用索引會致使全表掃描 is null ,is not null也沒法使用索引 like以通配符開頭 ("%abc") mysql索引失效會變成全表掃描 字符串補加引號索引失效 少用or，用它來鏈接時會索引失效
#建立測試數據
CREATE TABLE `staffs` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(24) NOT NULL DEFAULT '' COMMENT '姓名', `age` int(11) NOT NULL DEFAULT '0' COMMENT '年齡', `pos` varchar(20) NOT NULL DEFAULT '' COMMENT '職位', `add_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '入職時間', PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='員工記錄表' insert into staffs(name,age,pos,add_time) values ('z3',22,"manager",now()); insert into staffs(name,age,pos,add_time) values ('July',23,"dev",now()); insert into staffs(name,age,pos,add_time) values ('2000',23,"dev",now());
alter table staffs add index idx_staffs_nap(name,age,pos);
mysql> explain select * from staffs where name="July"; +----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 74 | const | 1 | Using where | +----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+ mysql> explain select * from staffs where name="July" and age="25"; +----+-------------+--------+------+----------------+----------------+---------+-------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+----------------+----------------+---------+-------------+------+-------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 78 | const,const | 1 | Using where | +----+-------------+--------+------+----------------+----------------+---------+-------------+------+-------------+
mysql> explain select * from staffs where name="July" and age="25" and pos="dev"; +----+-------------+--------+------+----------------+----------------+---------+-------------------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+--------+------+----------------+----------------+---------+-------------------+------+-------------+ | 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 140 | const,const,const | 1 | Using where | +----+-------------+--------+------+----------------+----------------+---------+-------------------+------+-------------+
mysql> explain select * from staffs where age="25" and pos="dev";
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
mysql> explain select * from staffs where pos="dev";
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
==>前綴生效原則，即最佳左前綴法則
若是索引了多列，查詢須要從索引的最左前列開始而且不跳過索引中的列
mysql> explain select * from staffs where name="July" and pos="dev";
+----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+
| 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 74 | const | 1 | Using where |
+----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+
==>中間跳過了一個，索引部分生效，單獨where name時key_len是74，如今依然是74。建議也別跳過
mysql> explain select * from staffs where left(name,4)="July";
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
==>不在索引列上作任何操做（計算、函數、類型轉換（自動或手動）），會致使索引失效而轉向全表掃描
mysql> explain select * from staffs where name="July" and age>25 and pos="dev";
+----+-------------+--------+-------+----------------+----------------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+----------------+----------------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | range | idx_staffs_nap | idx_staffs_nap | 78 | NULL | 1 | Using where |
+----+-------------+--------+-------+----------------+----------------+---------+------+------+-------------+
==>存儲引擎不能使用索引中範圍條件右邊的列,範圍以後全失效（age被用到了，可是用在了排序，而不是查找）
mysql> explain select name,age,pos from staffs where name="July" and age=25 and pos="manager";
+----+-------------+--------+------+----------------+----------------+---------+-------------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+----------------+---------+-------------------+------+--------------------------+
| 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 140 | const,const,const | 1 | Using where; Using index |
+----+-------------+--------+------+----------------+----------------+---------+-------------------+------+--------------------------+
mysql> explain select * from staffs where name="July" and age=25 and pos="manager";
+----+-------------+--------+------+----------------+----------------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+----------------+---------+-------------------+------+-------------+
| 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 140 | const,const,const | 1 | Using where |
+----+-------------+--------+------+----------------+----------------+---------+-------------------+------+-------------+
==>儘可能使用覆蓋索引，減小select *
mysql> explain select * from staffs where name !="July";
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | idx_staffs_nap | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
mysql> explain select * from staffs where name <>"July";
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | idx_staffs_nap | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
==>mysql在使用不等於（!=或<>）的時候沒法使用索引會致使全表掃描
mysql> explain select * from staffs where name is null;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
mysql> explain select * from staffs where name is not null;
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | idx_staffs_nap | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
==>is null ,is not null也沒法使用索引
mysql> explain select * from staffs where name like '%July';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | NULL | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
mysql> explain select * from staffs where name like 'July%';
+----+-------------+--------+-------+----------------+----------------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+----------------+----------------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | range | idx_staffs_nap | idx_staffs_nap | 74 | NULL | 1 | Using where |
+----+-------------+--------+-------+----------------+----------------+---------+------+------+-------------+
==>like以通配符開頭 ("%abc") mysql索引失效會變成全表掃描
如何解決這種須要使用的狀況呢
create table tbl_user (
id int(11) not null auto_increment,
name varchar(20) default null,
age int(11) default null,
email varchar(20) default null,
primary key (id)
) engine=innodb auto_increment=1 default charset=utf8;
insert into tbl_user(name,age,email)values('1aa1','21','b@163.com');
insert into tbl_user(name,age,email)values('2aa2','222','a@163.com');
insert into tbl_user(name,age,email)values('3aa3','265','c@163.com');
insert into tbl_user(name,age,email)values('4aa4','21','d@163.com');
進行以下代碼測試==>經過創建聯合索引，即覆蓋索引來解決
mysql> explain select name,age from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> explain select * from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> explain select name from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> explain select id from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> explain select age from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> explain select id,name,age from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> explain select * from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ mysql> create index idx_user_nameAge on tbl_user(name,age); Query OK, 0 rows affected (0.14 sec) ************************************* mysql> explain select name,age from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select id from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select name from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select age from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select id,name from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select id,name,age from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select name,age from tbl_user where name like "%aa%"; +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ | 1 | SIMPLE | tbl_user | index | NULL | idx_user_nameAge | 68 | NULL | 4 | Using where; Using index | +----+-------------+----------+-------+---------------+------------------+---------+------+------+--------------------------+ mysql> explain select * from tbl_user where name like "%aa%"; +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tbl_user | ALL | NULL | NULL | NULL | NULL | 4 | Using where | +----+-------------+----------+------+---------------+------+---------+------+------+-------------+
mysql> explain select * from staffs where name=2000;(隱形的數字轉換)
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | idx_staffs_nap | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
mysql> explain select * from staffs where name="2000";
+----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+
| 1 | SIMPLE | staffs | ref | idx_staffs_nap | idx_staffs_nap | 74 | const | 1 | Using where |
+----+-------------+--------+------+----------------+----------------+---------+-------+------+-------------+
==>字符串不加引號索引失效
mysql> explain select * from staffs where name='July' or name='z3';
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
| 1 | SIMPLE | staffs | ALL | idx_staffs_nap | NULL | NULL | NULL | 3 | Using where |
+----+-------------+--------+------+----------------+------+---------+------+------+-------------+
==>少用or，用它來鏈接時會索引失效
測試數據，建立聯合索引，觀察相關執行狀況 create table test03 (
id int primary key not null auto_increment,
c1 char(10),
c2 char(10),
c3 char(10),
c4 char(10),
c5 char(10)
); insert into test03(c1,c2,c3,c4,c5)values('a1','a2','a3','a4','a5'); insert into test03(c1,c2,c3,c4,c5)values('b1','b2','b3','b4','b5'); insert into test03(c1,c2,c3,c4,c5)values('c1','c2','c3','c4','c5'); insert into test03(c1,c2,c3,c4,c5)values('d1','d2','d3','d4','d5'); insert into test03(c1,c2,c3,c4,c5)values('e1','e2','e3','e4','e5'); create index inx_c1234 on test03(c1,c2,c3,c4);
mysql> explain select * from test03 where c1='a1';
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 31 | const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
mysql> explain select * from test03 where c1='a1' and c2='a2';
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 62 | const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
mysql> explain select * from test03 where c1='a1' and c2='a2' and c3='a3';
+----+-------------+--------+------+---------------+-----------+---------+-------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 93 | const,const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------------+------+-------------+
mysql> explain select * from test03 where c1='a1' and c2='a2' and c3='a3' and c4='a4';
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 124 | const,const,const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
mysql> explain select * from test03 where c1='a1' and c2='a2' and c4='a4' and c3='a3';
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 124 | const,const,const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
mysql> explain select * from test03 where c4='a4' and c3='a3' and c2='a2' and c1='a1';
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 124 | const,const,const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------------------+------+-------------+
==>MYSQL OPTIMIZER查詢優化器起的做用
mysql> explain select * from test03 where c1='a1' and c2='a2' and c3>'a3' and c4='a4';
==>範圍後索引失效，c3用在了排序
mysql> explain select * from test03 where c1='a1' and c2='a2' and c3>'a3' and c4='a4';
+----+-------------+--------+-------+---------------+-----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+-----------+---------+------+------+-------------+
| 1 | SIMPLE | test03 | range | inx_c1234 | inx_c1234 | 93 | NULL | 1 | Using where |
+----+-------------+--------+-------+---------------+-----------+---------+------+------+-------------+
mysql> explain select * from test03 where c1='a1' and c2='a2' and c4>'a4' and c3='a3';
+----+-------------+--------+-------+---------------+-----------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+-------+---------------+-----------+---------+------+------+-------------+
| 1 | SIMPLE | test03 | range | inx_c1234 | inx_c1234 | 124 | NULL | 1 | Using where |
+----+-------------+--------+-------+---------------+-----------+---------+------+------+-------------+
==>用到了4個，c4>'a4'範圍以後全失效，可是c4是最後一個，因此。
mysql> explain select * from test03 where c1='a1' and c2='a2' and c4='a4' order by c3;
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 62 | const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
==>c3用到了，可是用在了排序，而不是查找
mysql> explain select * from test03 where c1='a1' and c2='a2' order by c3;
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 62 | const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
==>效果同上
mysql> explain select * from test03 where c1='a1' and c2='a2' order by c4;
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-----------------------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 62 | const,const | 1 | Using where; Using filesort |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-----------------------------+
==>c3斷了,Using filesort
mysql> explain select * from test03 where c1='a1' and c5='a5' order by c2,c3;
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 31 | const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
==>只用c1字段排序，可是c2，c3用於排序，所以無Using filesort，若是後面order by c3,c4會出現
mysql> explain select * from test03 where c1='a1' and c2='a2' order by c3,c2;
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 62 | const,const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------------+------+-------------+
==>由於排序字段c2已是個常量，即c2是一個惟一值。
mysql> explain select * from test03 where c1='a1' and c4='a4' group by c2,c3;
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 31 | const | 1 | Using where |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-------------+
mysql> explain select * from test03 where c1='a1' and c4='a4' group by c3,c2;
+----+-------------+--------+------+---------------+-----------+---------+-------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+----------------------------------------------+
| 1 | SIMPLE | test03 | ref | inx_c1234 | inx_c1234 | 31 | const | 1 | Using where; Using temporary; Using filesort |
+----+-------------+--------+------+---------------+-----------+---------+-------+------+-----------------------------------------
==>group by基本上都須要排序，會有臨時表產生（分組以前必排序）
索引優化通常性建議
一、對於單鍵索引，儘可能選擇對當前query過濾性更好的索引
二、在選擇組合索引的時候，當前query中過濾性最好的字段在索引字段順序中，位置越靠前越好
三、在選擇組合索引的時候，儘可能選擇可能可以包含當前query中的where字句中更多字段的索引
四、儘量經過分析統計信息和調整query的寫法來達到選擇合適索引的目的
SQL語句查詢優化原則：小表驅動大表
#################原理######### select * from A where id in (select id from B) 等價於 for select id from B for select * from A where A.id=B.id 當B表的數據集必須小於A表的數據集時，用in優於exist select * from A where exists (select 1 from B where B.id=A.id) 等價於 for select * from A for select * from B where B.id=A.id 當A表的數據集小於B表的數據集時，用exist優於in（A與B表的ID字段應該創建索引） EXISTS select ... from table where exists(subquery) 該語法能夠理解爲，將主查詢的數據，放到子查詢中作條件驗證，根據驗證結果（TRUE或FALSE），來決定主查詢的數據結果是否得以保留 EXISTS(subquery）只返回TRUE或FALSE EXISTS(subquery）實際過程當中可能通過了優化而不是咱們理解上的逐條對比 EXISTS(subquery）每每也可使用條件表達式，其餘子查詢或者join來替代，何種最優須要具體分析
Order by優化
reate table tblA ( age int, birth timestamp not null ); insert into tblA(age,birth)values(22,now()); insert into tblA(age,birth)values(23,now()); insert into tblA(age,birth)values(24,now()); create index idx_ab on tblA(age,birth);
mysql> explain select * from tblA where age >20 order by age;
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
| 1 | SIMPLE | tblA | index | idx_ab | idx_ab | 9 | NULL | 3 | Using where; Using index |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
mysql> explain select * from tblA where age >20 order by birth;
+----+-------------+-------+-------+---------------+--------+---------+------+------+------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+------------------------------------------+
| 1 | SIMPLE | tblA | index | idx_ab | idx_ab | 9 | NULL | 3 | Using where; Using index; Using filesort |
+----+-------------+-------+-------+---------------+--------+---------+------+------+------------------------------------------+
mysql> explain select * from tblA where age >20 order by age,birth;
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
| 1 | SIMPLE | tblA | index | idx_ab | idx_ab | 9 | NULL | 3 | Using where; Using index |
+----+-------------+-------+-------+---------------+--------+---------+------+------+--------------------------+
mysql> explain select * from tblA order by age asc,birth desc;
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------+
| 1 | SIMPLE | tblA | index | NULL | idx_ab | 9 | NULL | 3 | Using index; Using filesort |
+----+-------------+-------+-------+---------------+--------+---------+------+------+-----------------------------+
==>order by默認升序
order by排序結論
Mysql支持FileSort和Index方式排序，Index方式效率高（掃描索引），並儘量的在索引列上完成排序操做，遵循索引創建的最佳左前綴原則
若是不在索引列上，filesort有兩種算法：雙路排序和單路排序 雙路排序
雙路排序：mysql4.1前使用雙路排序，掃描兩次磁盤，獲得數據。先讀取行指針和order by的列，進行排序，而後掃描已經排序好的列表，按照列表中的值從新從列表中讀取對應的數據輸出
單路排序：從磁盤讀取查詢須要的全部列，按照order by列在buffer對他們進行排序，而後掃描排序後的列表進行輸出，效率更快些，避免二次讀取數據，而且把隨機IO變成順序IO，可是須要更多的空間，由於它把每行都保存在內存中了
在sort_buffer中，方法B要比A暫用更多空間，因此有可能取出的數據總大小超出其容量，致使每次只能讀取其相應容量的數據，進行排序（建立tmp文件，多路合併），排完再取，再排，從而屢次IO，反而致使大量IO
提升order by速度
一、order by使用select *是大忌
當query字段大小總和小於max_lenth_for_sort_data並且字段不是text或blob類型時，會使用單路，不然使用多路
兩種算法都有可能超出sort_buffer容量，超出的話使用單路風險更大，因此能夠提升sort_buffer_size
二、嘗試提升sort_buffer_size
對兩種算法都有效率提高，要注意此參數針對每一個進程
三、嘗試提升max_lenth_for_sort_data
會增長使用改進算法的機率，可是入股哦過高，數據總量超過sort_buffer_size的機率就增大，明顯症狀是高的磁盤IO和低的cpu使用效率
order by知足兩種狀況會使用Index方式排序
一、order by語句使用索引最左前列
二、使用where子句與order by子句條件列組合知足索引最左前列
爲排序使用索引
Mysql兩種排序方式：文件排序和掃描有序索引排序
Mysql能爲排序和查詢使用相同的索引
key a_b_c(a,b,c)
order by使用索引前綴生效
order by a order by a,b order by a,b,c
order by a desc,b desc,c desc 要麼同升，要麼同降
若是where使用最左前綴定義爲常量，則order by能使用索引
where a = const order by b,c
where a = const and b =const order by c
where a = const and b>const order by b,c
不能使用索引進行排序
order by a asc,b desc,c desc 排序不一致
where g=const order by b,c 丟失a索引
where a=const order by c 丟失b索引
where a=const order by a,d d不是索引的一部分
where a in (...) order by b,c 對於排序來講，多個相等條件也是範圍查詢
group by關鍵字優化
實質是先排序後分組，遵守索引建的最佳左前綴
當沒法使用索引列，增大max_length_for_sort_data參數的設置+增大sort_buffer_size參數的設置
where高於having，能寫在where限定的條件就不要去having限定了
使用explain優化SQL語句的基本流程（通常讓開發去優化）
一、抓慢查詢，而後使用explain語句檢查索引執行狀況
a、mysql> show full processlist\G (現場抓）或#mysql -uroot -prootabcd -e "show processlist;" | grep -i select（效果更好） ,以後能夠用explain或profile分析
b、分析慢查詢日誌（平時運維）使用mysqlsla工具分析（自帶的mysqldumpslow比較簡陋）
long_query_time=2 超過2秒以上的查詢記錄下來
log-slow-queries=/data/slow.log
log_queries_not_using_indexes 沒走索引的也記錄
mysql> show variables like "%slow_query%";
+---------------------+-----------------------+
| Variable_name | Value |
+---------------------+-----------------------+
| slow_query_log | OFF |
| slow_query_log_file | /data/node80-slow.log |
+---------------------+-----------------------+
mysql> show variables like "%long_query%";
+-----------------+-----------+
| Variable_name | Value |
+-----------------+-----------+
| long_query_time | 10.000000 |
+-----------------+-----------+
mysql> show global status like "%Slow_que%";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Slow_queries | 0 |
+---------------+-------+
mysqldump分析工具
返回記錄集最多的10個sql mysqldumpslow -s r -t 10 slowfile | more
返回訪問次數最多的10個sql mysqldumpslow -s c -t 10 slowfile | more
按時間排序的前10條裏面含有作連接的sql mysqldumpslow -s t -t 10 -g "left join" slowfile | more
對slow.log,進行切割腳本分析 切割慢查詢(寫成腳本，使用mysqlsla分析),自帶mysqldumpslow工具
cd /data/ &&
/bin/mv slow.log /back/$(date +%F)_slow.log
mysqladmin -uroot -prootabcd flush-logs (也會刷新binlog)
mysqlsla /back/$(date +%F)_slow.log > new_/back/$(date +%F)_slow.log
mail -s "logname" mailadress </bakup/ new_/back/$(date +%F)_slow.log
二、explain語句檢查索引執行狀況
三、對須要建索引的列創建索引或者參數調優（最後才考慮調優）
使用explain優化sql語句場景案例
優化原由
一、網站慢，瀏覽慢，假定查出是數據庫問題
二、數據庫服務器查看uptime,負載很高（load average:8.01 6.30,5.58 超過核數數屬於高的）
三、登錄數據庫show full processlist===>再用explain 檢查語句是否走索引，查看建表語句或show index等，
四、慢查詢語句或日誌
五、select count(distinct列) from table 查看字段惟一數 根據這個和開發溝通建立相關索引，若是都很少考慮聯合索引，若是大表，建立須要幾分鐘，建議在業務低谷執行
mysql> show global variables like "profiling";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| profiling | OFF |
+---------------+-------+
注意以下問題
converting HEAP to MyISAM查詢結果太大，內存都不夠用了往磁盤上搬了
Creating tmp table 建立臨時表
Copying to tmp table on disk 把內存中的臨時表複製到磁盤，危險
locked
鎖
鎖是計算機協調多個進程或線程併發訪問某一資源的機制
根據操做類型分
讀鎖（共享鎖）：針對同一份數據，多個讀操做能夠同時進行而不會互相影響
寫鎖（排它鎖）：當前寫操做沒有完成前，它會阻斷其餘寫鎖和讀鎖
根據數據操做的力度
行鎖
表鎖
myisam表鎖分析
create table mylock( id int not null primary key auto_increment, name varchar(20) ) engine myisam;
insert into mylock(name) values('a');
insert into mylock(name) values('b');
insert into mylock(name) values('c');
insert into mylock(name) values('d');
insert into mylock(name) values('e');
mysql> show open tables; 查看哪些表被加鎖了 +--------------------+----------------------------------------------+--------+-------------+ | Database | Table | In_use | Name_locked | +--------------------+----------------------------------------------+--------+-------------+ | mysql | time_zone_transition_type | 0 | 0 | | performance_schema | events_waits_summary_global_by_event_name | 0 | 0 | | performance_schema | setup_timers | 0 | 0 | | performance_schema | events_waits_history_long | 0 | 0 | | gtms | tblA | 0 | 0 | | performance_schema | mutex_instances | 0 | 0 | | performance_schema | events_waits_summary_by_instance | 0 | 0 | | mysql | tables_priv | 0 | 0 | | gtms | mylock | 0 | 0 |
mysql> show status like "table%";
+-----------------------+-------+
| Variable_name | Value |
+-----------------------+-------+
| Table_locks_immediate | 54 |產生表鎖的次數，表示能夠當即獲取鎖的查詢次數，每當即獲取鎖值加1
| Table_locks_waited | 0 |出現表鎖爭用而發生的等待次數，（不能當即獲取的次數，每等待一次加1），此值高說明存在較嚴重的表鎖爭用
+-----------------------+-------+
加解鎖的方法
mysql> lock table mylock read;
mysql> lock table mylock write;
mysql> unlock tables;
進行測試
mysql> lock table mylock read; 增長讀鎖
mysql> update mylock set name='a2' where id=1;
ERROR 1099 (HY000): Table 'mylock' was locked with a READ lock and can't be updated 不能進行更新
mysql> select * from article;
ERROR 1100 (HY000): Table 'article' was not locked with LOCK TABLES 也不能讀其餘表
==>只能讀本身，不能改本身，也不能讀其餘表
==>再開一個sesson，嘗試update，會阻塞，等待解鎖後被執行。可想而知，效率呢？
mysql> unlock tables;
mysql> lock table mylock write;
mysql> update mylock set name='a2' where id=1;
mysql> select * from article; 不能讀其餘表
ERROR 1100 (HY000): Table 'article' was not locked with LOCK TABLES
==>再開個session，對mylock進行讀操做，阻塞（備註：測試時換成不一樣ID，由於第二次條件會從緩存得到，影響鎖效果演示）
mysql表鎖結論
一、讀鎖，不會阻塞其餘進程對同一表的讀請求，可是會阻塞對同一表的寫請求，只有釋放後，纔會執行其餘進程的操做
二、寫鎖，會阻塞其餘進程對同一表的讀寫操做，釋放後，纔會執行其餘進程的讀寫操做
==>簡而言之，讀會阻塞寫，可是不會阻塞讀，寫鎖則會把讀和寫都阻塞
此外，myisam的讀寫鎖調度室寫優先，這也是不適合做爲主表的引擎，由於寫鎖後，其餘線程不能作任何操做，大量的更新使查詢很可貴鎖，從而形成永遠阻塞
InnoDB與MyISAM最大不一樣點：支持事務及行級鎖（行鎖會變成表鎖，由於char類型沒加""）
1、原子性(Atomicity)：事務中的所有操做在數據庫中是不可分割的，要麼所有完成，要麼均不執行。 2、一致性(Consistency)：幾個並行執行的事務，其執行結果必須與按某一順序串行執行的結果相一致。 3、隔離性(Isolation)：事務的執行不受其餘事務的干擾，事務執行的中間結果對其餘事務必須是透明的。（未提交讀，已提交讀，可重複讀，可序列化4個級別） 好比--single-transaction 4、持久性(Durability)：對於任意已提交事務，系統必須保證該事務對數據庫的改變不被丟失，即便數據庫出現故障。一個事務一旦被提交，它對數據庫中的數據改變就是永久性的，若是出了錯誤，事務也不容許撤銷，只能經過「補償性事務」
間隙鎖
當咱們用範圍條件而不是相等條件檢索數據，並請求共享或排他鎖時，InnoDB會給符合條件的已有數據記錄的索引項加鎖； 對於鍵值在條件範圍內但並不存在的記錄，叫作「間隙（GAP)」，InnoDB也會對這個「間隙」加鎖，這種鎖機制就是所謂的間隙鎖（Next-Key鎖）。 例如 update table set b="444" where a>1 and a<6 (其中記錄4不存在) 當其餘session須要插入記錄4時，會被阻塞，直到以前的update語句commit後才能執行 InnoDB使用間隙鎖的目的，一方面是爲了防止幻讀，以知足相關隔離級別的要求， 對於上面的例子，要是不使用間隙鎖，若是其餘事務插入了a>1 and a<6 任何記錄，那麼本事務若是再次執行上述語句，就會發生幻讀； 很顯然，在使用範圍條件檢索並鎖定記錄時，InnoDB這種加鎖機制會阻塞符合條件範圍內鍵值的併發插入，這每每會形成嚴重的鎖等待 所以，在實際應用開發中，尤爲是併發插入比較多的應用，咱們要儘可能優化業務邏輯，儘可能使用相等條件來訪問更新數據，避免使用範圍條件。 InnoDB除了經過範圍條件加鎖時使用間隙鎖外，若是使用相等條件請求給一個不存在的記錄加鎖，InnoDB也會使用間隙鎖
如何鎖定一行
mysql>begin mysql>select * from tableA where a=8 for update;
xxxxxxxxxxxxxxxxxxxx msyql>commit;
分析行鎖爭奪狀況
mysql> show status like "innodb_row_lock%"; +-------------------------------+-------+ | Variable_name | Value | +-------------------------------+-------+ | Innodb_row_lock_current_waits | 0 |當前正在等待鎖定的數量 | Innodb_row_lock_time | 0 |從啓動到如今鎖定總時間長度 | Innodb_row_lock_time_avg | 0 |每次等待所花平均時間 | Innodb_row_lock_time_max | 0 |從系統啓動到如今等待最長一次所花時間 | Innodb_row_lock_waits | 0 |系統啓動後到如今總共等待的次數 +-------------------------------+-------+行鎖分析 儘量讓全部數據檢索都經過索引完成，避免無索引行鎖升級爲表鎖 合理設計索引，儘可能縮小鎖的範圍 儘量減小檢索條件，避免間隙鎖 儘可能控制事務大小，減小鎖定資源量和時間長度 儘量低級別事務隔離
相關文章
- 1. Mysql索引原理及索引優化
- 2. MySQL索引及優化
- 3. Mysql 索引及優化
- 4. mysql索引及優化
- 5. Mysql索引及優化
- 6. mysql 優化以及索引
- 7. MySQL 性能優化---索引及優化
- 8. mysql優化之索引篇
- 9. MySQL優化之索引(二)
- 10. Mysql優化之索引
- 更多相關文章...
- • SEO - 搜索引擎優化 - 網站建設指南
- • MySQL索引簡介 - MySQL教程
- • 漫談MySQL的鎖機制
- • Flink 數據傳輸及反壓詳解
相關標籤/搜索
每日一句
-
每一个你不满意的现在，都有一个你没有努力的曾经。
最新文章
- 1. 融合阿里雲，牛客助您找到心儀好工作
- 2. 解決jdbc(jdbctemplate)在測試類時不報錯在TomCatb部署後報錯
- 3. 解決PyCharm GoLand IntelliJ 等 JetBrains 系列 IDE無法輸入中文
- 4. vue+ant design中關於圖片請求不顯示的問題。
- 5. insufficient memory && Native memory allocation (malloc) failed
- 6. 解決IDEA用Maven創建的Web工程不能創建Java Class文件的問題
- 7. [已解決] Error: Cannot download ‘https://start.spring.io/starter.zip?
- 8. 在idea讓java文件夾正常使用
- 9. Eclipse啓動提示「subversive connector discovery」
- 10. 帥某-技巧-快速轉帖博主文章（article_content）
歡迎關注本站公眾號,獲取更多信息
相關文章
- 1. Mysql索引原理及索引優化
- 2. MySQL索引及優化
- 3. Mysql 索引及優化
- 4. mysql索引及優化
- 5. Mysql索引及優化
- 6. mysql 優化以及索引
- 7. MySQL 性能優化---索引及優化
- 8. mysql優化之索引篇
- 9. MySQL優化之索引(二)
- 10. Mysql優化之索引