MySQL-索引管理及執行計劃
1、索引介紹
1.一、索引做用
提供了相似於書中目錄的做用,目的是爲了優化查詢html
1.二、索引算法上分類
大的分類：mysql
B樹索引 Hash索引 R樹 Full text GIS #地圖類索引 ------------------------------- #B樹基於不一樣的查找算法分類： B-tree 如下兩種類型在範圍查詢方面提供了更好的性能(> < >= <=) B+Tree B*Tree
1.三、索引功能上的分類
1.3.一、輔助索引
1）輔助索引(S)怎麼構建B樹結構的?linux
- (1)索引是基於表中列(索引鍵)的值生成的B樹結構
- (2)首先提取此列全部的值,進行自動排序
- (3)將排好序的值,均勻的分佈到索引樹的葉子節點中(16K)
- (4)而後生成此索引鍵值所對應得後端數據頁的指針
- (5)生成枝節點和根節點,根據數據量級和索引鍵長度,生成合適的索引樹高度
2）輔助索引細分面試
- 1）普通的單列輔助索引（普通索引）
- 2）覆蓋索引(聯合索引)：多個列做爲索引條件,生成索引樹,理論上設計的好的,能夠減小大量的回表查詢
- 3）惟一索引:索引列的值都是惟一的.
1.3.二、彙集索引
1）前提redis
- (1)表中設置了主鍵,主鍵列就會自動被做爲彙集索引.
- (2)若是沒有主鍵,會選擇惟一鍵做爲彙集索引.
- (3)彙集索引必須在建表時纔有意義,通常是表的無關列(ID)
2）彙集索引(C)怎麼構建B樹結構的?算法
- (1) 在建表時,設置了主鍵列(ID)
- (2) 在未來錄入數據時,就會按照ID列的順序存儲到磁盤上.(咱們又稱之爲彙集索引組織表)
- (3) 將排好序的整行數據,生成葉子節點.能夠理解爲,磁盤的數據頁就是葉子節點
1.3.三、索引高度影響因數
1）數據量級, 解決方法:分表,分庫,分佈式sql
2）索引列值過長 , 解決方法:前綴索引mongodb
3）數據類型: 數據庫
變長長度字符串,使用了char,解決方案:變長字符串使用varcharvim
enum類型的使用enum ('山東','河北','黑龍江','吉林','遼寧','陝西'......)
1.3.四、B樹查找算法
2、索引基本管理
2.一、查看索引
mysql> desc city; +-------------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+----------+------+-----+---------+----------------+ | ID | int(11) | NO | PRI | NULL | auto_increment | | Name | char(35) | NO | | | | | CountryCode | char(3) | NO | MUL | | | | District | char(20) | NO | | | | | Population | int(11) | NO | | 0 | | +-------------+----------+------+-----+---------+----------------+ Field :列名字 key :有沒有索引,索引類型 PRI: 主鍵索引 UNI: 惟一索引 MUL: 輔助索引(單列,聯和,前綴) mysql> show index from city; +-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | city | 0 | PRIMARY | 1 | ID | A | 4188 | NULL | NULL | | BTREE | | | | city | 1 | CountryCode | 1 | CountryCode | A | 4188 | NULL | NULL | | BTREE | | | +-------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2.二、索引建立修改刪除
1）建立索引
alter table city add index idx_name(name); #方法一 create index idx_name1 on city(name); #方法二 show index from city;
2）刪除索引
alter table city drop index idx_name1; #直接刪除索引名稱便可
3）覆蓋索引(聯合索引)
alter table city add key idx_co_po(countrycode,population); #多個字段上創建索引 alter table city add index idx_co_po(countrycode,population); #多個字段上創建索引
4）前綴索引
alter table city add index idx_di(district(5)); #在前5個字符上創建索引
5）惟一索引
alter table city add unique index idx_uni1(name); ERROR 1062 (23000): Duplicate entry 'San Jose' for key 'idx_uni1' #惟一鍵衝突
3、執行計劃
3.一、執行計劃介紹
獲取到的是優化器選擇完成的,他認爲代價最小的執行計劃.
做用: 語句執行前,先看執行計劃信息,能夠有效的防止性能較差的語句帶來的性能問題.
3.二、select 獲取數據的方法
1. 全表掃描(應當儘可能避免,由於性能低)
2. 索引掃描
3. 獲取不到數據
3.三、執行計劃獲取及分析
獲取優化器選擇後的執行計劃：explain或者desc
explain select SQL_NO_CACHE * from test where name='AAA'\G #SQL_NO_CACHE的做用是禁止緩存查詢結果。
mysql> desc select * from city where countrycode='CHN'\G
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: city
partitions: NULL
type: ref
possible_keys: CountryCode
key: CountryCode
key_len: 3
ref: const
rows: 363
filtered: 100.00
Extra: NULL
---------------------------------------------------------------
#執行計劃相關信息分析：
table: city #查詢操做的表
possible_keys: CountryCode,idx_co_po #可能會走的索引
key: CountryCode #真正走的索引
type: ref #索引類型
Extra: Using index condition #額外信息
4、索引類型詳解 *****
以下爲索引類型，從左到右性能依次變好.
ALL #全表掃描 index #全索引掃描 range #索引範圍查詢 ref #輔助索引的等值查詢 eq_ref #多表鏈接的表,On的條件是主鍵或惟一鍵 system(const) #主鍵或惟一鍵的等值查詢 NULL #索引中掃描不到這個數據
在索引掃描類型方面，至少保證在range以上級別。
4.一、all: 全表掃描
desc select * from city;
desc select * from city where name like '%C%';
desc select * from city where name != 'CHN'; #或者<>
desc select * from city where countrycode not in ('CHN','USA');
#注意：生產中幾乎是沒有這種需求的。儘可能避免
4.二、index: 全索引掃描
須要掃描整個索引樹,獲取到想要數據,比ALL性能好,順序IO,能夠減小回表查詢
mysql> desc city; +-------------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+----------+------+-----+---------+----------------+ | ID | int(11) | NO | PRI | NULL | auto_increment | | Name | char(35) | NO | | | | | CountryCode | char(3) | NO | MUL | | | | District | char(20) | NO | | | | | Population | int(11) | NO | | 0 | | +-------------+----------+------+-----+---------+----------------+ mysql> desc select CountryCode from city; +----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+ | 1 | SIMPLE | city | NULL | index | NULL | CountryCode | 3 | NULL | 4188 | 100.00 | Using index | +----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
4.三、 range : 索引範圍查詢
4.3.一、狀況種類
> < >= <= in or like 'CH%' between and --------------------------------------------------- 注意： B+樹額外優化了 > < >= <= between and like 'CH%' in or沒法享受B+樹的額外優化,能夠用union all來替代
4.3.二、示例
mysql> desc select * from city where id<10;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | city | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 9 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
mysql> desc select * from city where countrycode in ('CHN','USA');
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | city | NULL | range | CountryCode | CountryCode | 3 | NULL | 637 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
mysql> desc select * from city where countrycode like 'CH%';
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | city | NULL | range | CountryCode | CountryCode | 3 | NULL | 397 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
4.3.三、優化案例：in=>union all
mysql> desc select * from city where countrycode in ('CHN','USA');
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | city | NULL | range | CountryCode | CountryCode | 3 | NULL | 637 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+-------------+---------+------+------+----------+-----------------------+
mysql> desc select * from city where countrycode='CHN' union all select * from city where countrycode='USA';
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
| 1 | PRIMARY | city | NULL | ref | CountryCode | CountryCode | 3 | const | 363 | 100.00 | NULL |
| 2 | UNION | city | NULL | ref | CountryCode | CountryCode | 3 | const | 274 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
4.四、 ref: 輔助索引的等值查詢
mysql> desc select * from city where countrycode = 'CHN'; +----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | city | NULL | ref | CountryCode | CountryCode | 3 | const | 363 | 100.00 | NULL | +----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+-------+
4.五、eq_ref
多表鏈接的表,On的條件是主鍵或惟一鍵
4.六、system 或 const
主鍵或惟一鍵的等值查詢
mysql> desc select * from city where id=10; +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | city | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL | +----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
4.七、NULL
索引中掃描不到這個數據
mysql> desc select * from city where id=5000; #id=5000不存在 +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+ | 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 matching row in const table | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
4.八、Extra字段：Using filesort問題
Extra字段：Using filesort #出現說明有問題，要優化 desc select * from city where countrycode='CHN' order by population desc limit 10; #contrycode上有索引，但population上是沒有索引的 mysql> desc select * from city where countrycode='CHN' order by population desc limit 10; +----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+---------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+---------------------------------------+ | 1 | SIMPLE | city | NULL | ref | CountryCode | CountryCode | 3 | const | 363 | 100.00 | Using index condition; Using filesort | +----+-------------+-------+------------+------+---------------+-------------+---------+-------+------+----------+---------------------------------------+ #解決思路： 索引能夠減小排序,能夠很大程度減小CPU時間 輔助索引 應用順序(優化器選擇的) 若是查詢條件:符合覆蓋索引的順序時,優先選擇覆蓋索引 不符合順序,優先會走where條件的索引 #解決方法：能夠在countrycode和population上創建聯合索引 mysql> alter table city add index idx_po(countrycode,population); mysql> desc select * from city where countrycode='CHN' order by population limit 10; +----+-------------+-------+------------+------+--------------------+--------+---------+-------+------+----------+-----------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+--------------------+--------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | city | NULL | ref | CountryCode,idx_po | idx_po | 3 | const | 363 | 100.00 | Using index condition | +----+-------------+-------+------------+------+--------------------+--------+---------+-------+------+----------+-----------------------+
4.九、explain使用場景
面試：咱們公司業務慢,請你從數據庫的角度分析緣由？ mysql出現性能問題,總結有兩種狀況: （1）應急性的慢：忽然夯住 應急狀況:數據庫hang(卡了,資源耗盡) 處理過程: (1)show processlist; #獲取到致使數據庫hang住的語句 (2)explain #分析SQL的執行計劃,有沒有走索引,索引的類型狀況 (3)建索引,改語句 （2）一段時間慢(持續性的): (1)記錄慢日誌slowlog,分析slowlog (2)explain 分析SQL的執行計劃,有沒有走索引,索引的類型狀況 (3)建索引,改語句
5、索引壓力測試
5.一、mysqlslap測試示例
mysqlslap工具介紹
mysqlslap來自於mariadb包，測試的過程默認生成一個mysqlslap的schema,生成測試表t1，查詢和插入測試數據，mysqlslap庫自動生成，若是已經存在則先刪除。用--only-print來打印實際的測試過程，整個測試完成後不會在數據庫中留下痕跡。
經常使用選項：
--auto-generate-sql, -a 自動生成測試表和數據，表示用mysqlslap工具本身生成的SQL腳原本測試併發壓力
--auto-generate-sql-load-type=type 測試語句的類型。表明要測試的環境是讀操做仍是寫操做仍是二者混合的。取值包括：read，key，write，update和mixed(默認)
--auto-generate-sql-add-auto-increment 表明對生成的表自動添加auto_increment列，從5.1.18版本開始支持
--number-char-cols=N, -x N 自動生成的測試表中包含多少個字符類型的列，默認1
--number-int-cols=N, -y N 自動生成的測試表中包含多少個數字類型的列，默認1
--number-of-queries=N 總的測試查詢次數(併發客戶數×每客戶查詢次數)
--query=name,-q 使用自定義腳本執行測試，例如能夠調用自定義的存儲過程或者sql語句來執行測試
--create-schema 表明自定義的測試庫名稱，測試的schema，MySQL中schema也就是database
--commint=N 多少條DML後提交一次
--compress, -C 如服務器和客戶端都支持壓縮，則壓縮信息
--concurrency=N, -c N 表示併發量，即模擬多少個客戶端同時執行select；可指定多個值，以逗號或者--delimiter參數指定值作爲分隔符
--engine=engine_name, -e engine_name 表明要測試的引擎，能夠有多個，用分隔符隔開
--iterations=N, -i N 測試執行的迭代次數，表明要在不一樣併發環境下，各自運行測試多少次
--only-print 只打印測試語句而不實際執行
--detach=N 執行N條語句後斷開重連
--debug-info, -T 打印內存和CPU的相關信息
測試示例：
1）單線程測試
[root@centos7 ~]# mysqlslap -a -uroot -p
Enter password:
Benchmark
Average number of seconds to run all queries: 0.004 seconds
Minimum number of seconds to run all queries: 0.004 seconds
Maximum number of seconds to run all queries: 0.004 seconds
Number of clients running queries: 1
Average number of queries per client: 0
2）多線程測試，使用--concurrency來模擬併發鏈接
[root@centos7 ~]# mysqlslap -uroot -p -a -c 500
Enter password:
Benchmark
Average number of seconds to run all queries: 3.384 seconds
Minimum number of seconds to run all queries: 3.384 seconds
Maximum number of seconds to run all queries: 3.384 seconds
Number of clients running queries: 500
Average number of queries per client: 0
3）同時測試不一樣的存儲引擎的性能進行對比
[root@centos7 ~]# mysqlslap -uroot -p -a --concurrency=500 --number-of-queries 1000 --iterations=5 --engine=myisam,innodb --debug-info
Enter password:
Benchmark
Running for engine myisam
Average number of seconds to run all queries: 0.192 seconds
Minimum number of seconds to run all queries: 0.187 seconds
Maximum number of seconds to run all queries: 0.202 seconds
Number of clients running queries: 500
Average number of queries per client: 2
Benchmark
Running for engine innodb
Average number of seconds to run all queries: 0.355 seconds
Minimum number of seconds to run all queries: 0.350 seconds
Maximum number of seconds to run all queries: 0.364 seconds
Number of clients running queries: 500
Average number of queries per client: 2
User time 0.33, System time 0.58
Maximum resident set size 22892, Integral resident set size 0
Non-physical pagefaults 46012, Physical pagefaults 0, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 31896, Involuntary context switches 0
4）執行一次測試，分別500和1000個併發，執行5000次總查詢
[root@centos7 ~]# mysqlslap -uroot -p -a --concurrency=500,1000 --number-of-queries 5000 --debug-info
Enter password:
Benchmark
Average number of seconds to run all queries: 3.378 seconds
Minimum number of seconds to run all queries: 3.378 seconds
Maximum number of seconds to run all queries: 3.378 seconds
Number of clients running queries: 500
Average number of queries per client: 10
Benchmark
Average number of seconds to run all queries: 3.101 seconds
Minimum number of seconds to run all queries: 3.101 seconds
Maximum number of seconds to run all queries: 3.101 seconds
Number of clients running queries: 1000
Average number of queries per client: 5
User time 0.84, System time 0.64
Maximum resident set size 83068, Integral resident set size 0
Non-physical pagefaults 139977, Physical pagefaults 0, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 31524, Involuntary context switches 3
5）迭代測試
[root@centos7 ~]# mysqlslap -uroot -p -a --concurrency=500 --number-of-queries 5000 --iterations=5 --debug-info
Enter password:
Benchmark
Average number of seconds to run all queries: 3.307 seconds
Minimum number of seconds to run all queries: 3.184 seconds
Maximum number of seconds to run all queries: 3.421 seconds
Number of clients running queries: 500
Average number of queries per client: 10
User time 2.18, System time 1.58
Maximum resident set size 74872, Integral resident set size 0
Non-physical pagefaults 327732, Physical pagefaults 0, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 73904, Involuntary context switches 3
1）模擬數據庫數據
drop database oldboy; create database oldboy charset utf8;
2）建立一個t1的表，而後導入50萬行數據
[root@db01 ~]# vim slap.sh
#!/bin/bash
HOSTNAME="localhost"
PORT="3306"
USERNAME="root"
PASSWORD="1"
DBNAME="oldboy"
TABLENAME="t1"
#create database
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} -e "drop database if exists ${DBNAME}"
create_db_sql="create database if not exists ${DBNAME}"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} -e "${create_db_sql}"
#create table
create_table_sql="create table if not exists ${TABLENAME}(stuid int not null primary key,stuname varchar(20) not null,stusex char(1)
not null,cardid varchar(20) not null,birthday datetime,entertime datetime,address varchar(100)default null)"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} ${DBNAME} -e "${create_table_sql}"
#insert data to table
i="1"
while [ $i -le 500000 ]
do
insert_sql="insert into ${TABLENAME} values($i,'alexsb_$i','1','110011198809163418','1990-05-16','2017-09-13','oldboyedu')"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} ${DBNAME} -e "${insert_sql}"
let i++
done
#select data
select_sql="select count(*) from ${TABLENAME}"
mysql -h ${HOSTNAME} -P${PORT} -u${USERNAME} -p${PASSWORD} ${DBNAME} -e "${select_sql}"
執行腳本：
sh slap.sh
或者直接source
drop database oldboy;
source /root/oldboy.sql
3）檢查數據可用性
mysql -uroot -p select count(*) from oldboy.t1;
4）使用mysqlslap來進行壓力測試
mysqlslap --defaults-file=/etc/my.cnf \ --concurrency=100 --iterations=1 --create-schema='oldboy' \ --query="select * from oldboy.t1 where stuname='alexsb_100'" engine=innodb \ --number-of-queries=2000 -uroot -pmysql -verbose
沒創建索引以前：
在查詢條件列上創建索引：alter table t1 add index idx_name(stuname);
5.二、其餘測試工具（待測試）
1）tpcc
2）sysbench
6、索引應用規範
主要根據公司的業務來創建合適的索引
- 產品的功能
- 用戶的行爲："熱"查詢語句，"熱"數據
6.一、創建索引的原則
爲了使索引的使用效率更高，在建立索引時，必須考慮在哪些字段上建立索引和建立什麼類型的索引。
1）建表時必定要有主鍵，通常是個無關列（必須）
2）選擇惟一鍵索引
惟一性索引的值是惟一的，能夠更快速的經過該索引來肯定某條記錄。
#優化方案：
(1) 若是非得使用重複值較多的列做爲查詢條件(例如:男女),能夠將表邏輯拆分
(2) 能夠將此列和其餘的查詢類,作聯和索引
#如何判斷索引列有多少是惟一值？
select count(*) from world.city;
select count(distinct countrycode) from world.city;
select count(distinct countrycode,population ) from world.city;
3）爲常常須要where 、ORDER BY、GROUP BY,join on等操做的字段創建索引，排序操做會浪費不少時間。注：若是常常做爲條件的列，重複值特別多，能夠創建聯合索引
4）使用前綴索引。若是索引字段的值很長，最好使用值的前綴來索引。
5）限制索引的數目
索引的數目不是越多越好。可能會產生的問題: (1) 每一個索引都須要佔用磁盤空間，索引越多，須要的磁盤空間就越大。 (2) 修改表時，對索引的重構和更新很麻煩。越多的索引，會使更新表變得很浪費時間。 (3) 優化器的負擔會很重,有可能會影響到優化器的選擇.
6）刪除再也不使用或者不多使用的索引(使用percona toolkit)
表中的數據被大量更新，或者數據的使用方式被改變後，原有的一些索引可能再也不須要。數據庫管理 員應當按期找出這些索引，將它們刪除，從而減小索引對更新操做的影響。
7）大表加索引,要在業務不繁忙期間操做
8）儘可能少在常常更新值的列上建索引
創建索引原則總結：
(1) 必需要有主鍵,若是沒有能夠作爲主鍵條件的列,建立無關列 (2) 常常作爲where條件列 order by group by join on, distinct 的條件(業務:產品功能+用戶行爲) (3) 最好使用惟一值多的列做爲索引,若是索引列重複值較多,能夠考慮使用聯合索引 (4) 列值長度較長的索引列,咱們建議使用前綴索引. (5) 下降索引條目,一方面不要建立沒用索引,不常使用的索引清理,使用percona toolkit工具 (6) 索引維護要避開業務繁忙期
6.二、 不走索引的狀況
1）沒有查詢條件，或者查詢條件沒有創建索引
select * from tab; #全表掃描 select * from tab where 1=1;
在業務數據庫中，特別是數據量比較大的表，是沒有全表掃描這種需求。
- 對用戶查看是很是痛苦的。
- 對服務器來說毀滅性的。
（1）select * from tab; SQL改寫成如下語句： selec * from tab order by price limit 10 ; #須要在price列上創建索引 （2）select * from tab where name='zhangsan' #name列沒有索引 改爲以下語句： 一、換成有索引的列做爲查詢條件 二、將name列創建索引
2）查詢的結果集，超過了總數行數25%，優化器默認沒有必要走索引
解決方法： 一、若是業務容許，可使用limit控制。 二、儘可能不要在mysql存放這個數據了，能夠放到redis裏面
3）索引自己失效，統計數據不真實
索引有自我維護的能力。對於表內容變化比較頻繁的狀況下，有可能會出現索引失效。通常是刪除重建
4）查詢條件使用函數在索引列上，或者對索引列進行運算，運算包括(+，-，*，/，! 等)
錯誤的例子：select * from test where id-1=9; 正確的例子：select * from test where id=10; 在索引列上使用算術運算，函數運算，子查詢時，可能不會走索引
5）隱式轉換致使索引失效
mysql> alter table tab add index inx_tel(telnum); mysql> desc tab; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | telnum | varchar(20) | YES | MUL | NULL | | #字段類型是varchar，同時創建了索引 +--------+-------------+------+-----+---------+-------+ mysql> select * from tab where telnum='1333333'; #查詢條件是字符串，走索引 +------+------+---------+ | id | name | telnum | +------+------+---------+ | 1 | a | 1333333 | +------+------+---------+ mysql> select * from tab where telnum=1333333; #查詢條件是數字，隱式轉換爲字符串 +------+------+---------+ | id | name | telnum | +------+------+---------+ | 1 | a | 1333333 | +------+------+---------+ mysql> explain select * from tab where telnum='1333333'; +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ | 1 | SIMPLE | tab | ref | inx_tel | inx_tel | 63 | const | 1 | Using index condition | +----+-------------+-------+------+---------------+---------+---------+-------+------+-----------------------+ mysql> explain select * from tab where telnum=1333333; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | tab | ALL | inx_tel | NULL | NULL | NULL | 2 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+
6）<> ，not in 不走索引
EXPLAIN SELECT * FROM teltab WHERE telnum <> '110';
EXPLAIN SELECT * FROM teltab WHERE telnum NOT IN ('110','119');
注意：
1. 單獨的>,<,in 有可能走，也有可能不走，和結果集有關，儘可能結合業務添加limit
2. or或in 儘可能改爲union
EXPLAIN SELECT * FROM teltab WHERE telnum IN ('110','119');
改寫成：
EXPLAIN SELECT * FROM teltab WHERE telnum='110'
UNION ALL
SELECT * FROM teltab WHERE telnum='119'
7）like "%_" 百分號在最前面不走
EXPLAIN SELECT * FROM teltab WHERE telnum LIKE '31%' #走range索引掃描 EXPLAIN SELECT * FROM teltab WHERE telnum LIKE '%110' #不走索引 %linux%類的搜索需求，可使用elasticsearch+mongodb 專門作搜索服務的數據庫產品
8）單獨引用聯合索引裏非第一位置的索引列.做爲條件查詢時不走索引
idx_a_b_c(a,b,c) #創建的聯合索引 走索引的狀況： where a b c where a b where a ============ 部分走索引 where a c where a c b ============ 不走索引 where c where b where bc where cb where ca where cba
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