Subject: 圣光照耀联盟—PostgreSQL临时表的创建与使用过程

From: Quan Zongliang <[email-masked]>
Date: Tue, 28 Jun 2016 09:34:28 +0800

原文发表于
http://my.oschina.net/quanzl/blog/701657

1、临时表的创建与普通表没什么区别，都是通过 DefineRelation 函数 （定义于 
src/backend/commands/tablecmds.c），临时表具有属性 relation->relpersistence == 
RELPERSISTENCE_TEMP

#define		  RELPERSISTENCE_PERMANENT	'p'		/* regular table */
#define		  RELPERSISTENCE_UNLOGGED	'u'		/* unlogged permanent table */
#define		  RELPERSISTENCE_TEMP		't'		/* temporary table */
2、首先为临时表创建临时schema，命名为 "pg_temp_%d", 
MyBackendId，也就是以客户端后台进程的PID为后缀，保证各进程不冲突。（消耗 
OID）
函数 InitTempTableNamespace定义于 src/backend/catalog/namespace.c

3、随后的创建过程与普通表区别不大，仍然消耗 OID

4、我们直接跳到缓存的分配看：

bool		isLocalBuf = SmgrIsTemp(smgr);

     if (isLocalBuf)
     {
         bufHdr = LocalBufferAlloc(smgr, forkNum, blockNum, &found);
         if (found)
             pgBufferUsage.local_blks_hit++;
         else
             pgBufferUsage.local_blks_read++;
     }
这里 isLocalBuf 就是临时表的意思

#define SmgrIsTemp(smgr) \
     RelFileNodeBackendIsTemp((smgr)->smgr_rnode)

/*
  * Augmenting a relfilenode with the backend ID provides all the 
information
  * we need to locate the physical storage.  The backend ID is 
InvalidBackendId
  * for regular relations (those accessible to more than one backend), 
or the
  * owning backend's ID for backend-local relations.  Backend-local 
relations
  * are always transient and removed in case of a database crash; they are
  * never WAL-logged or fsync'd.
  */
typedef struct RelFileNodeBackend
{
     RelFileNode node;
     BackendId	backend;
} RelFileNodeBackend;

5、临时表缓存

     if (LocalBufHash == NULL)
         InitLocalBuffers();
这里就是文档 Chapter 18. Server Configuration，18.4.1. Memory 里边关于 temp_buffers 
的说明：The setting can be changed within individual sessions, but only 
before the first use of temporary tables within the session; subsequent 
attempts to change the value will have no effect on that session.
第一次使用时才会初始化，随后再修改将不起作用。

--------------------------------------------
权宗亮

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