[20171206]rman与truncate.txt
[20171206]rman与truncate.txt
--//昨天下班在回家的路上,突然想起以前遇到的问题,就是truncate表后,rman做备份时会备份多少truncate表的信息,
--//当时在itpub上,还存在讨论,就是rman会备份空block吗?参考链接:http://www.itpub.net/thread-2050864-9-1.html
--//zergduan兄还开了SR,答复如下:
终于有回复了
经过dump的确认，unused block compression 特性是在extent一级，而不是在block级启用：
-------------------------
BEFORE : RelFno: 7, BeginBlock: 128, Flag: 0, First: 50, Free: 63438
BEFORE : FFFFFFFFFFFF0300 0000000000000000 0000000000000000 0000000000000000
AFTER : RelFno: 7, BeginBlock: 128, Flag: 0, First: 1, Free: 63487
AFTER : 0100000000000000 0000000000000000 0000000000000000 0000000000000000
RMAN read that there is one extent to be backup from the table. and I will backup all the blocks. It does not read at BLOCK level.
......
unused block compression doesn't kick in unless all the extents are unused. Granularity can't be changed to a block.
---------------------------
truncate之后，该对象仍然存在，所以，部分extent(比如头部)中仍然有一些metadata，这样的extent被认为非空，会被整体备份出来。
由于truncate 并不删除数据，所以，该块上仍然保留有您测试的字符串等。所以，通过strings命令可以看到这部分数据。
该问题的重点，我想应该是，最后验证了 unused block compression 的粒度是在extent级别，不是block。
--//是否跟rman input memory buffer有关.通过例子来来说明:
1.环境:
SCOTT@book> @ &r/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
$ cat d2.txt
drop tablespace t01 including contents and datafiles;
drop tablespace t02 including contents and datafiles;
drop tablespace t03 including contents and datafiles;
drop tablespace t04 including contents and datafiles;
drop tablespace t05 including contents and datafiles;
CREATE TABLESPACE T01 DATAFILE '/mnt/ramdisk/book/T01.dbf' SIZE 6M AUTOEXTEND OFF LOGGING ONLINE EXTENT MANAGEMENT LOCAL AUTOALLOCATE BLOCKSIZE 8K SEGMENT SPACE MANAGEMENT AUTO FLASHBACK ON;
CREATE TABLESPACE T02 DATAFILE '/mnt/ramdisk/book/T02.dbf' SIZE 6M AUTOEXTEND OFF LOGGING ONLINE EXTENT MANAGEMENT LOCAL AUTOALLOCATE BLOCKSIZE 8K SEGMENT SPACE MANAGEMENT AUTO FLASHBACK ON;
CREATE TABLESPACE T03 DATAFILE '/mnt/ramdisk/book/T03.dbf' SIZE 6M AUTOEXTEND OFF LOGGING ONLINE EXTENT MANAGEMENT LOCAL AUTOALLOCATE BLOCKSIZE 8K SEGMENT SPACE MANAGEMENT AUTO FLASHBACK ON;
CREATE TABLESPACE T04 DATAFILE '/mnt/ramdisk/book/T04.dbf' SIZE 6M AUTOEXTEND OFF LOGGING ONLINE EXTENT MANAGEMENT LOCAL AUTOALLOCATE BLOCKSIZE 8K SEGMENT SPACE MANAGEMENT AUTO FLASHBACK ON;
CREATE TABLESPACE T05 DATAFILE '/mnt/ramdisk/book/T05.dbf' SIZE 6M AUTOEXTEND OFF LOGGING ONLINE EXTENT MANAGEMENT LOCAL AUTOALLOCATE BLOCKSIZE 8K SEGMENT SPACE MANAGEMENT AUTO FLASHBACK ON;
create table t01 tablespace t01 as select rownum id ,to_char(rownum,'FM000000')||lpad('A',26,'A') name from dual connect by level<=1e5;
create table t02 tablespace t02 as select rownum id ,to_char(rownum,'FM000000')||lpad('B',26,'B') name from dual connect by level<=1e5;
create table t03 tablespace t03 as select rownum id ,to_char(rownum,'FM000000')||lpad('C',26,'C') name from dual connect by level<=1e5;
create table t04 tablespace t04 as select rownum id ,to_char(rownum,'FM000000')||lpad('D',26,'D') name from dual connect by level<=1e5;
create table t05 tablespace t05 as select rownum id ,to_char(rownum,'FM000000')||lpad('E',26,'E') name from dual connect by level<=1e5;
alter system checkpoint;
alter system checkpoint;
alter system checkpoint;
2.测试:
--//建立如上表空间:
RMAN> CONFIGURE CHANNEL 1 DEVICE TYPE DISK clear;
old RMAN configuration parameters:
CONFIGURE CHANNEL 1 DEVICE TYPE DISK RATE 128 K;
old RMAN configuration parameters are successfully deleted
RMAN> report schema ;
Report of database schema for database with db_unique_name BOOK
List of Permanent Datafiles
===========================
File Size(MB) Tablespace RB segs Datafile Name
---- -------- -------------------- ------- ------------------------
1 760 SYSTEM *** /mnt/ramdisk/book/system01.dbf
2 940 SYSAUX *** /mnt/ramdisk/book/sysaux01.dbf
3 1075 UNDOTBS1 *** /mnt/ramdisk/book/undotbs01.dbf
4 256 USERS *** /mnt/ramdisk/book/users01.dbf
5 346 EXAMPLE *** /mnt/ramdisk/book/example01.dbf
6 40 TEA *** /mnt/ramdisk/book/tea01.dbf
7 6 T01 *** /mnt/ramdisk/book/T01.dbf
8 6 T02 *** /mnt/ramdisk/book/T02.dbf
9 6 T03 *** /mnt/ramdisk/book/T03.dbf
10 6 T04 *** /mnt/ramdisk/book/T04.dbf
11 6 T05 *** /mnt/ramdisk/book/T05.dbf
List of Temporary Files
=======================
File Size(MB) Tablespace Maxsize(MB) Tempfile Name
---- -------- -------------------- ----------- --------------------
1 414 TEMP 32767 /mnt/ramdisk/book/temp01.dbf
SCOTT@book> truncate table t01 ;
Table truncated.
--//truncate表T01.测试备份看看:
RMAN> backup datafile 7 format '/home/oracle/backup/T1_%U';
Starting backup at 2017-12-06 08:37:56
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=80 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/mnt/ramdisk/book/T01.dbf
channel ORA_DISK_1: starting piece 1 at 2017-12-06 08:37:57
channel ORA_DISK_1: finished piece 1 at 2017-12-06 08:37:58
piece handle=/home/oracle/backup/T1_h7sle5j5_1_1 tag=TAG20171206T083757 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2017-12-06 08:37:58
$ strings -t d T1_h7sle5j5_1_1 | grep 'AAAAA'|wc
9978 26939 440924
$ strings -t d T1_h7sle5j5_1_1 | grep 'AAAAAA' |head -1
1082678 K 000174AAAAAAAAAAAAAAAAAAAAAAAAAA,
$ strings -t d T1_h7sle5j5_1_1 | grep 'AAAAAA' |tail -1
1581019 009807AAAAAAAAAAAAAAAAAAAAAAAAAA
--//可以确定在备份集合中记录是那些.因为T02表结构是一样的.通过T02也可以确定备份记录的块号.
SCOTT@book> select rowid,t02.* from t02 where id in (174,9807);
ROWID ID NAME
------------------ ---------- --------------------------------
AAAWLjAAIAAAACDACt 174 000174BBBBBBBBBBBBBBBBBBBBBBBBBB
AAAWLjAAIAAAAC/AAA 9807 009807BBBBBBBBBBBBBBBBBBBBBBBBBB
SCOTT@book> @ &r/rowid AAAWLjAAIAAAACDACt
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90851 8 131 173 0x2000083 8,131 alter system dump datafile 8 block 131 ;
SCOTT@book> @ &r/rowid AAAWLjAAIAAAAC/AAA
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90851 8 191 0 0x20000BF 8,191 alter system dump datafile 8 block 191 ;
--//总共191-131+1=61,加上前面3块,就是64块,也就是 64*8192 = 524288,524288/1024=512K.
--//我以前一直的理解truncate后会备份一些块,仅仅备份一个extent,这样仅仅备份64K(8块).而实际上是512K.
--//为什么不是1M,而是512K?????
3.测试:
--//测试备份4个文件作为备份集的情况:
RMAN> backup datafile 7,8,9,10 format '/home/oracle/backup/T1234_%U';
Starting backup at 2017-12-06 08:53:53
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=94 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/mnt/ramdisk/book/T01.dbf
input datafile file number=00008 name=/mnt/ramdisk/book/T02.dbf
input datafile file number=00009 name=/mnt/ramdisk/book/T03.dbf
input datafile file number=00010 name=/mnt/ramdisk/book/T04.dbf
channel ORA_DISK_1: starting piece 1 at 2017-12-06 08:53:54
channel ORA_DISK_1: finished piece 1 at 2017-12-06 08:53:55
piece handle=/home/oracle/backup/T1234_h8sle6h2_1_1 tag=TAG20171206T085353 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2017-12-06 08:53:55
$ strings -t d T1234_h8sle6h2_1_1 | grep 'AAAAA' |head -1
4179254 K 000174AAAAAAAAAAAAAAAAAAAAAAAAAA,
$ strings -t d T1234_h8sle6h2_1_1 | grep 'AAAAA' |tail -1
4677595 009807AAAAAAAAAAAAAAAAAAAAAAAAAA
$ strings -t d T1234_h8sle6h2_1_1 | grep 'AAAAA' |wc
9978 26939 440924
--//与备份1个数据文件一样,不展开分析了.本来还想测试备份5个数据文件的.
--//因为备份1个文件仅仅备份512K(在truncate后),我开始的理解应该是1M.why?
--//不过我的理解还是和rman input memory buffer有关.
4.测试:
SCOTT@book> drop tablespace t01 including contents and datafiles;
Tablespace dropped.
CREATE TABLESPACE t01 DATAFILE
'/mnt/ramdisk/book/T01.dbf' SIZE 6M AUTOEXTEND OFF
LOGGING
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO
FLASHBACK ON;
--//建立UNIFORM SIZE 1M.
create table t01 tablespace t01 as select rownum id ,to_char(rownum,'FM000000')||lpad('A',26,'A') name from dual connect by level<=1e5;
SCOTT@book> create table t01 tablespace t01 as select rownum id ,to_char(rownum,'FM000000')||lpad('A',26,'A') name from dual connect by level<=1e5;
Table created.
SCOTT@book> truncate table t01 ;
Table truncated.
RMAN> backup datafile 7 format '/home/oracle/backup/T1x_%U';
Starting backup at 2017-12-06 09:02:19
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/mnt/ramdisk/book/T01.dbf
channel ORA_DISK_1: starting piece 1 at 2017-12-06 09:02:19
channel ORA_DISK_1: finished piece 1 at 2017-12-06 09:02:20
piece handle=/home/oracle/backup/T1x_h9sle70r_1_1 tag=TAG20171206T090219 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2017-12-06 09:02:20
$ strings -t d T1x_h9sle70r_1_1 | grep 'AAAAA' |wc
100000 270040 4424269
$ strings -t d T1x_h9sle70r_1_1 | grep 'AAAAA' |head -1
1090870 K 000174AAAAAAAAAAAAAAAAAAAAAAAAAA,
$ strings -t d T1x_h9sle70r_1_1 | grep 'AAAAA' |tail -1
6021081 c< 099859AAAAAAAAAAAAAAAAAAAAAAAAAA
$ strings -t d T1x_h9sle70r_1_1 | grep 'AAAAA' |wc
100000 270040 4424269
--//昏,全部备份.看来没有这么简单.
SCOTT@book> drop table t01 purge ;
Table dropped.
SCOTT@book> create table t01 tablespace t01 as select rownum id ,to_char(rownum,'FM000000')||lpad('A',26,'A') name from dual connect by level<=1e5;
Table created.
SCOTT@book> select rowid,t01.* from t01 where id in (174,99859);
ROWID ID NAME
------------------ ---------- --------------------------------
AAAWLuAAHAAAACEACt 174 000174AAAAAAAAAAAAAAAAAAAAAAAAAA
AAAWLuAAHAAAALdAAA 99859 099859AAAAAAAAAAAAAAAAAAAAAAAAAA
SCOTT@book> @ &r/rowid AAAWLuAAHAAAACEACt
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90862 7 132 173 0x1C00084 7,132 alter system dump datafile 7 block 132 ;
SCOTT@book> @ &r/rowid AAAWLuAAHAAAALdAAA
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90862 7 733 0 0x1C002DD 7,733 alter system dump datafile 7 block 733 ;
SCOTT@book> column PARTITION_NAME noprint
SCOTT@book> select * from dba_extents where segment_name='T01';
OWNER SEGMENT_NAME SEGMENT_TYPE TABLESPACE_NAME EXTENT_ID FILE_ID BLOCK_ID BYTES BLOCKS RELATIVE_FNO
------ ------------ ------------ --------------- --------- ---------- ---------- ---------- ---------- ------------
SCOTT T01 TABLE T01 0 7 128 1048576 128 7
SCOTT T01 TABLE T01 1 7 256 1048576 128 7
SCOTT T01 TABLE T01 2 7 384 1048576 128 7
SCOTT T01 TABLE T01 3 7 512 1048576 128 7
SCOTT T01 TABLE T01 4 7 640 1048576 128 7
--//难道truncate后rman备份会8个extents吗?
5.测试:
SCOTT@book> drop tablespace t01 including contents and datafiles;
Tablespace dropped.
CREATE TABLESPACE t01 DATAFILE
'/mnt/ramdisk/book/T01.dbf' SIZE 21M AUTOEXTEND OFF
LOGGING
EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M
BLOCKSIZE 8K
SEGMENT SPACE MANAGEMENT AUTO
FLASHBACK ON;
--//建立UNIFORM SIZE 1M.
create table t01 tablespace t01 as select rownum id ,to_char(rownum,'FM000000')||lpad('A',26,'A') name from dual connect by level<=4e5;
SCOTT@book> select * from dba_extents where segment_name='T01';
OWNER SEGMENT_NAME SEGMENT_TYPE TABLESPACE_NAME EXTENT_ID FILE_ID BLOCK_ID BYTES BLOCKS RELATIVE_FNO
------ ------------ ------------ --------------- --------- ---------- ---------- ---------- ---------- ------------
SCOTT T01 TABLE T01 0 7 128 1048576 128 7
SCOTT T01 TABLE T01 1 7 256 1048576 128 7
SCOTT T01 TABLE T01 2 7 384 1048576 128 7
SCOTT T01 TABLE T01 3 7 512 1048576 128 7
SCOTT T01 TABLE T01 4 7 640 1048576 128 7
SCOTT T01 TABLE T01 5 7 768 1048576 128 7
SCOTT T01 TABLE T01 6 7 896 1048576 128 7
SCOTT T01 TABLE T01 7 7 1024 1048576 128 7
SCOTT T01 TABLE T01 8 7 1152 1048576 128 7
SCOTT T01 TABLE T01 9 7 1280 1048576 128 7
SCOTT T01 TABLE T01 10 7 1408 1048576 128 7
SCOTT T01 TABLE T01 11 7 1536 1048576 128 7
SCOTT T01 TABLE T01 12 7 1664 1048576 128 7
SCOTT T01 TABLE T01 13 7 1792 1048576 128 7
SCOTT T01 TABLE T01 14 7 1920 1048576 128 7
SCOTT T01 TABLE T01 15 7 2048 1048576 128 7
SCOTT T01 TABLE T01 16 7 2176 1048576 128 7
SCOTT T01 TABLE T01 17 7 2304 1048576 128 7
SCOTT T01 TABLE T01 18 7 2432 1048576 128 7
19 rows selected.
SCOTT@book> truncate table t01 ;
Table truncated.
RMAN> backup datafile 7 format '/home/oracle/backup/T1y_%U';
Starting backup at 2017-12-06 09:17:52
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00007 name=/mnt/ramdisk/book/T01.dbf
channel ORA_DISK_1: starting piece 1 at 2017-12-06 09:17:52
channel ORA_DISK_1: finished piece 1 at 2017-12-06 09:17:53
piece handle=/home/oracle/backup/T1y_hbsle7u0_1_1 tag=TAG20171206T091752 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 2017-12-06 09:17:53
$ strings -t d T1y_hbsle7u0_1_1 | grep 'AAAAA' |head -1
1090870 K 000174AAAAAAAAAAAAAAAAAAAAAAAAAA,
$ strings -t d T1y_hbsle7u0_1_1 | grep 'AAAAA' |tail -1
9445337 [L 169075AAAAAAAAAAAAAAAAAAAAAAAAAA
$ strings -t d T1y_hbsle7u0_1_1 | grep 'AAAAA' |wc
169242 456998 7484144
SCOTT@book> drop table t01 purge ;
Table dropped.
SCOTT@book> create table t01 tablespace t01 as select rownum id ,to_char(rownum,'FM000000')||lpad('A',26,'A') name from dual connect by level<=4e5;
Table created.
SCOTT@book> select rowid,t01.* from t01 where id in (174,169075,169242);
ROWID ID NAME
------------------ ---------- --------------------------------
AAAWL0AAHAAAACEACt 174 000174AAAAAAAAAAAAAAAAAAAAAAAAAA
AAAWL0AAHAAAAR/AAA 169075 169075AAAAAAAAAAAAAAAAAAAAAAAAAA
AAAWL0AAHAAAAR/ACn 169242 169242AAAAAAAAAAAAAAAAAAAAAAAAAA
SCOTT@book> @ &r/rowid AAAWL0AAHAAAACEACt
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90868 7 132 173 0x1C00084 7,132 alter system dump datafile 7 block 132 ;
SCOTT@book> @ &r/rowid AAAWL0AAHAAAAR/AAA
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90868 7 1151 0 0x1C0047F 7,1151 alter system dump datafile 7 block 1151
SCOTT@book> @ &r/rowid AAAWL0AAHAAAAR/ACn
OBJECT FILE BLOCK ROW ROWID_DBA DBA TEXT
---------- ---------- ---------- ---------- -------------------- -------------------- ----------------------------------------
90868 7 1151 167 0x1C0047F 7,1151 alter system dump datafile 7 block 1151
--//昏,确实是8个extents.
--//也就是truncate后rman备份信息的量最大是8个extents.有点小疑问,我的测试使用的磁盘空间是连续的,如果离散是否也是这样,不测试了浪费时间.
[20171206]rman与truncate.txt的更多相关文章
- [20171206]rman与truncate2.txt
[20171206]rman与truncate2.txt --//上午测试发现truncate的表在做rman备份时还要做8个extents的备份.--//不知道自己的猜测是否正确,选择一个使用UNI ...
- [20171130]关于rman备份疑问.txt
[20171130]关于rman备份疑问.txt --//前面测试太乱,重新做一些rman as copy相关测试. 1.环境:SCOTT@book> @ &r/ver1PORT_STR ...
- [20171031]rman xxx Failure.txt
[20171031]rman xxx Failure.txt --//简单测试 List Failure, Advise Failure and Repair Failure命令在11g下,也许以后工 ...
- [20190517]rman tape compressed.txt
[20190517]rman tape compressed.txt --//前一段测试rman选择压缩备份时,如果备份单个数据文件小于64M-3*8K时,先建立大小64M文件,最后会回收到自己的备份 ...
- oracle数据库rman备份计划及恢复
1.rman完全恢复的前提条件:历史的datafile,controlfile和spfile备份,加上完整的archivelog和完好的redolog. 2.rman备份脚本: a.RMAN 0级备份 ...
- 转 rman 恢复报错
###sample 1 原因是新库起了FRA 区,FRA 区的旧文件属于之前的归档日志文件产生,这样会导致无法识别的问题. 解决办法,清空FRA或者恢复时候不启用FRA. RMAN RESTORE ...
- 官方文档 恢复备份指南二 Getting Started with RMAN
本章对RMAN进行基本的熟悉和了解 1.Overview of the RMAN Environment RMAN运行时需要的最小环境: target database ...
- RMAN详细教程（二）：备份、检查、维护、恢复
RMAN详细教程(一):基本命令代码 一.创建增量备份 增量备份级别为0-4,但为方便备份管理,oracle建议只限于0级和1级. 1.差异增量备份(differential incremental ...
- Oracle自动备份数据
一. Oracle自动备份单表一个月数据 方式一:三步处理(建批处理文件,写sql文件,设置任务计划) 1. 第一步:建立一个批处理文件 @echo off Set OrclSid=orcl Set ...
随机推荐
- Python模块——PrettyTable 模块
简介 PrettyTable 是python中的一个第三方库,可用来生成美观的ASCII格式的表格,十分实用. 安装 pip install prettytable 示例 从已有文件创建 CSV fr ...
- 使用 Portainer UI 管理 Docker 主机
Docker 使用命令行的方式来管理有时候并没有那么直观,可以使用 Portainer 的 UI 来管理 Docker 主机和 Docker Swarm 集群. 安装 Portainer 环境:cen ...
- 通向全栈之路（6）—无密码ssh连接
(1)在用户目录下新增一个 .ssh文件 mkdir .ssh (2)生成公钥.私钥 ssh-keygen -t rsa -b 4096 -C "mail@xxx.com" (3) ...
- Repeater 控件的嵌套使用
Repeater 控件的嵌套使用 ItemDataBound:数据绑定的时候(正在进行时)发生,多用在Repeater控件嵌套,对子Repeater控件进行数据绑定及模板列中统计列的计算处理等 ...
- MathJax: 让前端支持数学公式
文章图片存储在GitHub,网速不佳的朋友,请看<MathJax:让前端支持数学公式> 或者 来我的技术小站 godbmw.com 1. 必须要说 1.1 开发背景 博主使用Vue开发的个 ...
- Spring基于纯注解方式的使用
经过上篇xml与注解混合方式,对注解有了简单额了解,上篇的配置方式极大地简化了xml中配置,但仍有部分配置在xml中进行,接下来我们就通过注解的方式将xml中的配置用注解的方式实现,并最终去掉xml配 ...
- SQL Server 连接(内连接，外连接，完全连接，交叉连接，联合)
1.连接 有时候需要将连个表的数据合并成一个结果集来显示.为了解决这个问题,就需要用到JOIN连接. 2.内部连接 内部连接根据一个或几个共同的字段将记录匹配到一起.内部连接仅仅返回那些存在字段匹配的 ...
- csharp: sum columns or rows in a dataTable
DataTable dt = setData(); // Sum rows. //foreach (DataRow row in dt.Rows) //{ // int rowTotal = 0; / ...
- 【读书笔记】iOS-自定义URL Scheme注意事项
如果两个不同的应用注册了同样的URL Scheme,那么后安装的应用会响应符合这种协议格式的URL. 如果你的应用的iPhone和iPad版是各自独立的(即不是Universal类型的),那么你就不应 ...
- React 入门学习笔记整理（二）—— JSX简介与语法
先看下这段代码: import React from 'react'; //最终渲染需要调用ReactDOM库,将jsx渲染都页面中 import ReactDOM from 'react-dom'; ...