db file sequential read:

创新互联是专业的阿克塞哈萨克族自治网站建设公司，阿克塞哈萨克族自治接单;提供成都网站设计、网站建设、外贸网站建设,网页设计,网站设计,建网站,PHP网站建设等专业做网站服务;采用PHP框架,可快速的进行阿克塞哈萨克族自治网站开发网页制作和功能扩展;专业做搜索引擎喜爱的网站,专业的做网站团队,希望更多企业前来合作!

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：

1. 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；

2. 优化sql语句，减少不必要的块读取；

文章名称：Oracle等待事件之dbfilesequentialread
地址分享：http://www.cqwzjz.cn/article/jdhigj.html