Sequential和Scattered的含义
之前，我对Sequential和Scattered的理解是Oracle读取IO的顺序不同，今天又讨论起这个问题，另外一种解释是两者在内存中的保存方式有差异，Sequential read在内存中是连续的，而Scattered read是离散的。以下解释来自于Oracle文档：
Scattered read：
This event signifies that the user process is reading buffers into the SGA buffer cache and is waiting for a physical I/O call to return. A db file scattered read issues a scatter-read to read the data into multiple discontinuous memory locations. A scattered read is usually a multiblock read. It can occur for a fast full scan (of an index) in addition to a full table scan.
Sequential read：
This event signifies that the user process is reading buffers into the SGA buffer cache and is waiting for a physical I/O call to return. This call differs from a scattered read, because a sequential read is reading data into contiguous memory space. A sequential read is usually a single-block read.
这也从另外一个方面解释了Sequential和Scattered的含义。
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其实我很迷惑内存连续的确定含义，目前也没想到手段去证明。
从物理磁盘读数据块到内存中，数据块的物理地址映射到内存地址。连续的物理块原则上分配的内存也是连续的。
“sequential”还有个含义就是读的时间从index到table是sequentially的
“scattered”就是把内容scatter到buffer cache
@BlueBird03
Oracle的Buffer Cache分配机制无法保证这一点, 或者说保证这一点代价很大..
相关的内容可以参考. Oracle读取操作相关等待事件的命名理由