[20191218]降序索引疑问4.txt

--//前几天优化一个项目,我发现许多表里面有有隐含字段,一般开发很少建立函数索引.我自己检查发现里面存在大量的降序索引.
--//我感觉有点奇怪,为什么开发要建立大量降序索引有什么好处呢?

--//我在链接http://www.itpub.net/thread-2122088-1-1.html里面问这个问题,sqysl的解答给了我很好的提示,我通过例子说明使用降
--//序索引的一点点好处,通过例子说明:

1.环境:
SCOTT@book> @ ver1
PORT_STRING         VERSION        BANNER
------------------- -------------- ----------------------------------------------------------------------------
x86_64/Linux 2.4.xx 11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SCOTT@book> alter system set pga_aggregate_target=4G;
System altered.

SCOTT@book> create table t (id1 number,id2 number,vc varchar2(50));
Table created.

create index i_t_id1_id2 on t (id1,id2);
create index i_t_id1desc_id2desc on t (id1 desc,id2 desc);
create index i_t_id1desc_id2 on t (id1 desc,id2 );
create index i_t_id1_id2desc on t (id1 ,id2 desc );

SCOTT@book> insert into t select rownum,rownum,lpad('a',50,'a') from dual connect by level<=1e6;
1000000 rows created.

SCOTT@book> commit ;
Commit complete.

--//分析略.
SCOTT@book> select index_name,index_type,blevel,leaf_blocks,distinct_keys from dba_indexes where owner=user and table_name='T';
INDEX_NAME                     INDEX_TYPE                      BLEVEL LEAF_BLOCKS DISTINCT_KEYS
------------------------------ --------------------------- ---------- ----------- -------------
I_T_ID1_ID2                    NORMAL                               2        2623       1000000
I_T_ID1DESC_ID2DESC            FUNCTION-BASED NORMAL                2        5877       1000000
I_T_ID1DESC_ID2                FUNCTION-BASED NORMAL                2        5618       1000000
I_T_ID1_ID2DESC                FUNCTION-BASED NORMAL                2        2753       1000000
--//无论那种倒序索引,索引都比正序索引大,特别是第1个字段选择desc的情况.因为我插入的字段是自增字段.
--//倒序索引分裂都是50-50分裂.

2.测试例子:
SCOTT@book> alter session set statistics_level = all;
Session altered.

SCOTT@book> select * from (select * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
       ID1        ID2 VC
---------- ---------- --------------------------------------------------
   1000000    1000000 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999999     999999 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999998     999998 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999997     999997 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999996     999996 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999995     999995 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999994     999994 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999993     999993 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999992     999992 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
    999991     999991 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
10 rows selected.

SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  20hb1uyyqx4tf, child number 0
-------------------------------------
select * from (select * from t where id1<=1e6 order by id1 desc,id2 )
where rownum<=10
Plan hash value: 404407004
-------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name            | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                 |      1 |        |       |     4 (100)|          |     10 |00:00:00.01 |       6 |
|*  1 |  COUNT STOPKEY                |                 |      1 |        |       |            |          |     10 |00:00:00.01 |       6 |
|   2 |   VIEW                        |                 |      1 |     10 |   530 |     4   (0)| 00:00:01 |     10 |00:00:00.01 |       6 |
|   3 |    TABLE ACCESS BY INDEX ROWID| T               |      1 |   1000K|    58M|     4   (0)| 00:00:01 |     10 |00:00:00.01 |       6 |
|*  4 |     INDEX RANGE SCAN          | I_T_ID1DESC_ID2 |      1 |     10 |       |     3   (0)| 00:00:01 |     10 |00:00:00.01 |       4 |
-------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$2 / from$_subquery$_001@SEL$1
   3 - SEL$2 / T@SEL$2
   4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(ROWNUM<=10)
   4 - access("T"."SYS_NC00004$">=HEXTORAW('3BFDFF')  AND "T"."SYS_NC00004$" IS NOT NULL)
       filter(SYS_OP_UNDESCEND("T"."SYS_NC00004$")<=1000000)
32 rows selected.
--//逻辑读仅仅6个.而其它2个索引都无法达到这样的效果.

select * from (select /*+ index(t I_T_ID1DESC_ID2DESC) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
select * from (select /*+ index_desc(t I_T_ID1DESC_ID2DESC) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;

select * from (select /*+ index(t I_T_ID1_ID2) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
select * from (select /*+ index_desc(t I_T_ID1_ID2) */ * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=10;
--//结果不在贴出.大家可以自行测试.

3.继续测试:

SCOTT@book> alter index I_T_ID1DESC_ID2 invisible;
Index altered.

SCOTT@book> select * from (select  * from t where id1<=1e6 order by id1 desc,id2 ) where rownum<=1;
       ID1        ID2 VC
---------- ---------- --------------------------------------------------
   1000000    1000000 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  2r7j4a5gdhpnj, child number 0
-------------------------------------
select * from (select  * from t where id1<=1e6 order by id1 desc,id2 )
where rownum<=1

Plan hash value: 3299198703

----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation               | Name | Starts | E-Rows |E-Bytes|E-Temp | Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |      |      1 |        |       |       | 17500 (100)|          |      1 |00:00:00.38 |    9285 |     74 |       |       |          |
|*  1 |  COUNT STOPKEY          |      |      1 |        |       |       |            |          |      1 |00:00:00.38 |    9285 |     74 |       |       |          |
|   2 |   VIEW                  |      |      1 |   1000K|    50M|       | 17500   (1)| 00:03:30 |      1 |00:00:00.38 |    9285 |     74 |       |       |          |
|*  3 |    SORT ORDER BY STOPKEY|      |      1 |   1000K|    58M|    69M| 17500   (1)| 00:03:30 |      1 |00:00:00.38 |    9285 |     74 |    65M|  2806K|          |
|*  4 |     TABLE ACCESS FULL   | T    |      1 |   1000K|    58M|       |  2744   (1)| 00:00:33 |   1000K|00:00:00.12 |    9285 |     74 |       |       |          |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------

1 - SEL$1
   2 - SEL$2 / from$_subquery$_001@SEL$1
   3 - SEL$2
   4 - SEL$2 / T@SEL$2

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter(ROWNUM<=1)
   3 - filter(ROWNUM<=1)
   4 - filter("ID1"<=1000000)
32 rows selected.
--//oracle选择全表扫描.

SCOTT@book> alter index I_T_ID1DESC_ID2 visible;
Index altered.

4.再继续测试:
--//测试第1个字段正序,第2个字段倒序的情况.
SCOTT@book> select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=5;
       ID1        ID2 VC
---------- ---------- --------------------------------------------------
         1          1 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         2          2 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         3          3 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         4          4 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         5          5 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  8f5yj8c3frpnc, child number 0
-------------------------------------
select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc )
where rownum<=5
Plan hash value: 2787951352
-------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name            | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                 |      1 |        |       |     4 (100)|          |      5 |00:00:00.01 |       6 |
|*  1 |  COUNT STOPKEY                |                 |      1 |        |       |            |          |      5 |00:00:00.01 |       6 |
|   2 |   VIEW                        |                 |      1 |      5 |   265 |     4   (0)| 00:00:01 |      5 |00:00:00.01 |       6 |
|   3 |    TABLE ACCESS BY INDEX ROWID| T               |      1 |   1000K|    58M|     4   (0)| 00:00:01 |      5 |00:00:00.01 |       6 |
|*  4 |     INDEX RANGE SCAN          | I_T_ID1_ID2DESC |      1 |      5 |       |     3   (0)| 00:00:01 |      5 |00:00:00.01 |       4 |
-------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$2 / from$_subquery$_001@SEL$1
   3 - SEL$2 / T@SEL$2
   4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(ROWNUM<=5)
   4 - access("ID1"<=1000000)
31 rows selected.
--//可以发现很好第使用I_T_ID1_ID2DESC索引.逻辑读也很小,仅仅6.

SCOTT@book> alter index I_T_ID1_ID2DESC invisible;
Index altered.

select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=1;
SCOTT@book> select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=5;
       ID1        ID2 VC
---------- ---------- --------------------------------------------------
         1          1 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         2          2 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         3          3 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         4          4 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         5          5 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  8f5yj8c3frpnc, child number 0
-------------------------------------
select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc )
where rownum<=5

Plan hash value: 2145689175

-----------------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name            | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
-----------------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |                 |      1 |        |       |     4 (100)|          |      5 |00:00:00.01 |       6 |      1 |
|*  1 |  COUNT STOPKEY                 |                 |      1 |        |       |            |          |      5 |00:00:00.01 |       6 |      1 |
|   2 |   VIEW                         |                 |      1 |      5 |   265 |     4   (0)| 00:00:01 |      5 |00:00:00.01 |       6 |      1 |
|   3 |    TABLE ACCESS BY INDEX ROWID | T               |      1 |   1000K|    58M|     4   (0)| 00:00:01 |      5 |00:00:00.01 |       6 |      1 |
|*  4 |     INDEX RANGE SCAN DESCENDING| I_T_ID1DESC_ID2 |      1 |      5 |       |     3   (0)| 00:00:01 |      5 |00:00:00.01 |       4 |      1 |
-----------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$2 / from$_subquery$_001@SEL$1
   3 - SEL$2 / T@SEL$2
   4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(ROWNUM<=5)
   4 - access("T"."SYS_NC00004$" IS NOT NULL AND "T"."SYS_NC00004$">=HEXTORAW('3BFDFF') )
       filter(SYS_OP_UNDESCEND("T"."SYS_NC00004$")<=1000000)
32 rows selected.

--//你会发现一个奇特现象,oracle会使用"倒过来"的一个索引,也就是id1 desc,id2 asc的索引.

SCOTT@book> alter index I_T_ID1DESC_ID2 invisible;
Index altered.

SCOTT@book> select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc ) where rownum<=5;
       ID1        ID2 VC
---------- ---------- --------------------------------------------------
         1          1 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         2          2 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         3          3 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         4          4 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
         5          5 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  8f5yj8c3frpnc, child number 0
-------------------------------------
select * from (select  * from t where id1<=1e6 order by id1 ,id2 desc )
where rownum<=5

Plan hash value: 3299198703

----------------------------------------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation               | Name | Starts | E-Rows |E-Bytes|E-Temp | Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT        |      |      1 |        |       |       | 17500 (100)|          |      5 |00:00:00.41 |    9279 |   9269 |       |       |          |
|*  1 |  COUNT STOPKEY          |      |      1 |        |       |       |            |          |      5 |00:00:00.41 |    9279 |   9269 |       |       |          |
|   2 |   VIEW                  |      |      1 |   1000K|    50M|       | 17500   (1)| 00:03:30 |      5 |00:00:00.41 |    9279 |   9269 |       |       |          |
|*  3 |    SORT ORDER BY STOPKEY|      |      1 |   1000K|    58M|    69M| 17500   (1)| 00:03:30 |      5 |00:00:00.41 |    9279 |   9269 |  2048 |  2048 | 2048  (0)|
|*  4 |     TABLE ACCESS FULL   | T    |      1 |   1000K|    58M|       |  2744   (1)| 00:00:33 |   1000K|00:00:00.15 |    9279 |   9269 |       |       |          |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$2 / from$_subquery$_001@SEL$1
   3 - SEL$2
   4 - SEL$2 / T@SEL$2
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter(ROWNUM<=5)
   3 - filter(ROWNUM<=5)
   4 - filter("ID1"<=1000000)
32 rows selected.
--//选择的是全表扫描.

SCOTT@book> alter index i_t_id1desc_id2desc invisible;
Index altered.

SCOTT@book> select * from (select  * from t where id1<=1e6 order by id1 desc ,id2 desc ) where rownum<=1;
       ID1        ID2 VC
---------- ---------- --------------------------------------------------
   1000000    1000000 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

SCOTT@book> @ dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  66z6w3qc77jwp, child number 0
-------------------------------------
select * from (select  * from t where id1<=1e6 order by id1 desc ,id2
desc ) where rownum<=1
Plan hash value: 3873686303

----------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                      | Name        | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers |
----------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT               |             |      1 |        |       |     4 (100)|          |      1 |00:00:00.01 |       4 |
|*  1 |  COUNT STOPKEY                 |             |      1 |        |       |            |          |      1 |00:00:00.01 |       4 |
|   2 |   VIEW                         |             |      1 |      1 |    53 |     4   (0)| 00:00:01 |      1 |00:00:00.01 |       4 |
|   3 |    TABLE ACCESS BY INDEX ROWID | T           |      1 |   1000K|    58M|     4   (0)| 00:00:01 |      1 |00:00:00.01 |       4 |
|*  4 |     INDEX RANGE SCAN DESCENDING| I_T_ID1_ID2 |      1 |      1 |       |     3   (0)| 00:00:01 |      1 |00:00:00.01 |       3 |
----------------------------------------------------------------------------------------------------------------------------------------
--//也能很好的选择正序索引.

5.总结:
--//通过以上测试,仅仅一种特殊的情况才需要建立倒序索引.存在id1 desc,id2 asc的情况下,建议完全可反过来建立(特别对于自增序列字段),
--//就是反过来建立索引id1 ,id2 desc,这样的索引占用磁盘空间更小.
--//其它情况我看不到降序索引的优势.

[20191218]降序索引疑问4.txt的更多相关文章

  1. [20181124]关于降序索引问题3.txt

    [20181124]关于降序索引问题3.txt --//链接:blog.itpub.net/267265/viewspace-2221425/,探讨降序索引中索引的键值.--//实际上使用函数sys_ ...

  2. [20181124]关于降序索引问题2.txt

    [20181124]关于降序索引问题2.txt --//链接:blog.itpub.net/267265/viewspace-2221425/,探讨降序索引中索引的键值.--//实际上使用函数sys_ ...

  3. [20190910]关于降序索引问题5.txt

    [20190910]关于降序索引问题5.txt --//测试了索引TERM使用0xfe表示,回想到以前遇到降序索引的特殊字符编码问题,现在可是忘得一干二净.--//现在想想当时自己怎么猜测出来的,^_ ...

  4. MySQL 8 新特性之降序索引

    MySQL 8.0终于支持降序索引了.其实,从语法上,MySQL 4就支持了,但正如官方文档所言,"they are parsed but ignored",实际创建的还是升序索引 ...

  5. MySQL8新增降序索引

    MySQL8新增降序索引 桃花坞里桃花庵,桃花庵里桃花仙.桃花仙人种桃树,又摘桃花卖酒钱. 一.MySQL5.7 降序索引 MySQL在语法上很早就已经支持降序索引,但实际上创建的却仍然是升序索引,如 ...

  6. 谈谈MySQL中的降序索引 order by id DESC / ASC

    今天这篇主要讲order by 语句中的多个字段asc desc的问题.mysql5中,索引存储的排序方式是ASC的,没有DESC的索引.现在能够理解为啥order by 默认是按照ASC来排序的了吧 ...

  7. Shell练习 统计单词个数，降序排列

    原文:https://leetcode.com/problems/word-frequency/ Write a bash script to calculate the frequency of e ...

  8. 选取两个有序数组中最大的K个值，降序存入另一个数组中

    原题: 假设有两个有序的整型数组int *a1, int *a2,长度分别为m和n.试用C语言写出一个函数选取两个数组中最大的K个值(K可能大于m+n)写到int *a3中,保持a3降序,并返回a3实 ...

  9. [20190423]那个更快的疑问3.txt

    [20190423]那个更快的疑问3.txt --//前一阵子,做了11g在单表单条记录唯一索引扫描的测试,摘要如下:--//参考链接:http://blog.itpub.net/267265/vie ...

随机推荐

  1. OAuth 2.0 概念及授权流程梳理

    本文可以转载,但请注明出处https://www.cnblogs.com/hellxz/p/oauth2_process.html OAuth2 的概念 OAuth是一个关于授权的开放网络标准,OAu ...

  2. 【RN - 基础】之React Native组件的生命周期

    下图描述了React Native中组件的生命周期: 从上图中可以看到,React Native组件的生命周期可以分为初始化阶段.存在阶段和销毁阶段. 实例化阶段 实例化阶段是React Native ...

  3. CentOS 7 安装 bind 服务 实现内网DNS

    目录 安装 配置 服务管理 测试 安装 废话不多说,直接安装 yum install -y bind bind-utils 配置 [root@jenkins named]# rpm -ql bind ...

  4. SpringBoot写一个登陆注册功能，和期间走的坑

    文章目录 前言 1. 首先介绍项目的相关技术和工具: 2. 首先创建项目 3. 项目的结构 3.1实体类: 3.2 Mapper.xml 3.3 mapper.inteface 3.4 Service ...

  5. 对python的初步了解

    一,Python简介 Python 是一个高层次的结合了解释性.编译性.互动性和面向对象的脚本语言. Python 的设计具有很强的可读性,相比其他语言经常使用英文关键字,其他语言的一些标点符号,它具 ...

  6. 常用tab选项卡代码

    <div class="box"> <ul> <li class="one">课程介绍</li> <li& ...

  7. 大型情感剧集Selenium：6_selenium中的免密登陆与cookie操作

    网站登录 现在各大平台在反爬虫功能上,可谓花样繁多.总结下来按照破解成功率排名,最高的是滑动解锁.其次是验证码数字.之后是一次点击对应的汉字,最后是想12306之前那种反人类的让你说那些是奶糖吧,哈哈 ...

  8. 【LiteOS】LiteOS移植常见问题

    发现很多人在LiteOS的移植过程中总会遇到一些问题,现在简单做一些总结.后续有新的问题提再继续补充. 1.CMSIS版本导致的问题 问题现象一般如下图所示,编译后报错,Undefined symbo ...

  9. 区块链学习笔记：DAY01 区块链的技术原理

    其实很早之前就听过区块链,也看过有关区块链的介绍,那个时候的理解主要还是一句话:分布式记账 然后开始关注比特币,听了有几年了,对于其来历.用途其实一直都是一知半解. 这次的课算是第一次以一个学员的身份 ...

  10. gitbook 入门教程之增强版 edit-link-plus 编辑此页插件

    Gitbook plugin add "edit this page" link, and intelligent recognition to Github or Gitlab ...