圣光照耀联盟—PostgreSQL临时表的创建与使用过程

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
神州飞象（北京）数据科技有限公司
我们的力量源自最先进的开源数据库PostgreSQL
zongliang.quan@postgresdata.com