日期：2016年2月29日

Oracle笔记（7）——Oracle进程，server进程和background进程

Oracle手册上对于Oracle process的定义：

A unit of execution that runs the Oracle database code. The process execution architecture depends on the operating system. Oracle processes include server processes and background processes.

Oracle process即是运行Oracle数据库的代码，它分成server process和background process两种。

Server process的定义：

An Oracle process that communicates with a client process and Oracle Database to fulfill user requests. The server processes are associated with a database instance, but are not part of the instance.

Server process可以看做是连接数据库client进程和数据库之间的桥梁，用来处理client的请求。虽然同Instance相关联，但不属于具体的Instance。

Background process的定义：

A process that consolidates functions that would otherwise be handled by multiple Oracle programs running for each client process. The background processes asynchronously perform I/O and monitor other Oracle processes.

Background process用来做实际的I/O操作和监控其它Oracle进程。

参考资料：
Glossary。

 

Oracle笔记（6）——SGA

Oracle手册上对于SGA（System Global Area）的定义：

The SGA is a read/write memory area that, along with the Oracle background processes, form a database instance. All server processes that execute on behalf of users can read information in the instance SGA. Several processes write to the SGA during database operation.

Note: The server and background processes do not reside within the SGA, but exist in a separate memory space.

Each database instance has its own SGA. Oracle Database automatically allocates memory for an SGA at instance startup and reclaims the memory at instance shutdown.

SGA和Oracle后台进程一起组成了数据库的Instance。每个Instance拥有自己的SGA，Instance启动时会自动分配SGA，退出时会回收SGA。SGA包含下列组成部分：

✓ Shared pool
✓ Database buffer cache
✓ Redo log buffer
✓ Large pool
✓ Java pool
✓ Streams pool

参考资料：
Oracle 12c For Dummies；
Overview of the System Global Area (SGA)。

 

Go语言的函数返回多个值

以下摘自The Go Programming Language：

Often, functions use these additional results to indicate some kind of error, either by returning an error as in the call to os.Open, or a bool, usually called ok. If a map lookup, type assertion, or channel receive appears in an assignment in which two results are expected, each produces an additional boolean result:
v, ok = m[key] // map lookup
v, ok = x.(T) // type assertion
v, ok = <-ch // channel receive
As with variable declarations, we can assign unwanted values to the blank identifier:
_, err = io.Copy(dst, src) // discard byte count
_, ok = x.(T)

map查找，类型断言和从channel接收数据都会返回两个值，其中一个是表示成功或失败的布尔值。同变量定义一样，不想要的值可以赋给_。