Oracle 等待事件之 db file sequential read

25-05-01 1

最近很多小伙伴都在问Oracle等待事件之dbfilesequentialread这两个问题，那么本篇文章就来给大家详细解答一下，同时本文还将给你拓展$file=isset($file)&&$file

最近很多小伙伴都在问Oracle 等待事件之 db file sequential read这两个问题，那么本篇文章就来给大家详细解答一下，同时本文还将给你拓展$file=isset($file) && $file?$file:''index''这段代码什么意思？、$file=isset($file) && $file?$file:''index''这段代码什么意思？该怎么处理、(Tcl -eval:) couldn''t read file "C:Swarm-2.2-java/lib/tcl8.4//word.tcl": no such file or directory while executing、adb 传输文件的权限问题——remote couldn''t create file: Read-only file system等相关知识，下面开始了哦！

本文目录一览：

Oracle 等待事件之 db file sequential read
$file=isset($file) && $file?$file:''index''这段代码什么意思？
$file=isset($file) && $file?$file:''index''这段代码什么意思？该怎么处理
(Tcl -eval:) couldn''t read file "C:Swarm-2.2-java/lib/tcl8.4//word.tcl": no such file or directory while executing
adb 传输文件的权限问题——remote couldn''t create file: Read-only file system

Oracle 等待事件之 db file sequential read

db file sequential read:

官网解释：

This event signifies that the user process is reading a buffer into the SGA buffer cache and is waiting for a physical I/O call to return. A sequential read is a single-block read.

Single block I/Os are usually the result of using indexes. Rarely, full table scan calls Could get truncated to a single block call because of extent boundaries, or buffers present in the buffer cache. These waits would also show up as db file sequential read.

Check the following V$SESSION_WAIT parameter columns:

P1: The absolute file number
P2: The block being read
P3: The number of blocks (should be 1)

On a healthy system, physical read waits should be the biggest waits after the idle waits. However, also consider whether there are db file sequential reads on a large data warehouse that should be seeing mostly full table scans with parallel query.

figure 10-1 depicts the differences between the following wait events:

db file sequential read (single block read into one SGA buffer)
db file scattered read (multiblock read into many discontinuous SGA buffers)
direct read (single or multiblock read into the PGA, bypassing the SGA)

figure 10-1 Scattered Read, Sequential Read, and Direct Path Read

解释：

db sequential read 此事件表示用户进程正在将缓冲区读入SGA缓冲区高速缓存并正在等待物理I / O调用返回。顺序读取是单块读取。

单块I / O通常是使用索引的结果。

全表扫描调用可能会因缓冲区高速缓存存在的范围边界或缓冲区而被截断的单块调用。这些等待也将显示为db file sequential read。

db file sequential read的优化方法：

从读取开始，增加SGA中buffer cache的大小，避免每次都从硬盘中去读数；
优化sql语句，减少不必要的块读取；

$file=isset($file) && $file?$file:''index''这段代码什么意思？

$file=isset($file) && $file?$file:''index'';
上面代码什么意思？最好能举个例子，上面实际执行了什么？

回复讨论(解决方案)

判断是否存在$file变量，如果存在则取值为$file 如果不存在则为index

? : 三目运算符
等价于
if(isset($file) && $file){
$file=$file;
}else{
$file=''index'';
}

? : 三元运算符 (?)问号前面是判断条件如果条件为真则取:(冒号)前面的值如果判断条件为假则取:(冒号)后面的值

$file=isset($file) && $file?$file:''index''这段代码什么意思？该怎么处理

$file=isset($file) && $file?$file:''index''这段代码什么意思？
$file=isset($file) && $file?$file:''index'';
上面代码什么意思？最好能举个例子，上面实际执行了什么？
------解决方案--------------------
判断是否存在$file变量，如果存在则取值为$file 如果不存在则为index
------解决方案--------------------
? : 三目运算符
等价于
if(isset($file) && $file){
$file=$file;
}else{
$file=''index'';
}
------解决方案--------------------
? : 三元运算符 (?)问号前面是判断条件如果条件为真则取:(冒号)前面的值如果判断条件为假则取:(冒号)后面的值

(Tcl -eval:) couldn''t read file

(Tcl -eval:) couldn''t read file "C:Swarm-2.2-java/lib/tcl8.4//word.tcl": no such file or directory while executing

(Tcl -eval:) couldn''t read file "C:Swarm-2.2-java/lib/tcl8.4//word.tcl": no such file or directory
while executing
"source C:\Swarm-2.2-java/lib/tcl8.4//word.tcl"
while evaluating: source C:\Swarm-2.2-java/lib/tcl8.4//word.tcl

各位大神们，这个问题怎么解决啊？？？

adb 传输文件的权限问题——remote couldn''t create file: Read-only file system

adb root

adb remount

提示错误，于是按照搜索的结果：

执行adb disable-verity

adb reboot

再次 adb root

adb remount

adb push即可使用

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