PostgreSQL数据库RelationAM——relation related routines

src/backend/access/common/relation.c 包含关系表例程，包含访问table、index等关系表，通过该文件提供的api，主要功能是为了从Relcache中获取relationId对应的RelationData结构体。This file contains relation_ routines that implement access to relations (tables, indexes, etc). Support that’s specific to subtypes of relations should go into their respective files, not here.

relation_open函数通过relation OID打开任何relation。a “relation” is anything with a pg_class entry. relation_open函数执行流程：

1. 如果lockmode需要加锁，则调用LockRelationOid进行加锁
2. 调用RelationIdGetRelation获取relationId对应的RelationData结构体，检查RelationData结构体是否合法
3. 如果是temporary relation，需要MyXactFlags |= XACT_FLAGS_ACCESSEDTEMPNAMESPACE改变标志位。

Relation relation_open(Oid relationId, LOCKMODE lockmode) {
	Relation	r;
	Assert(lockmode >= NoLock && lockmode < MAX_LOCKMODES);
	
	if (lockmode != NoLock) LockRelationOid(relationId, lockmode);
	
	r = RelationIdGetRelation(relationId);

	if (!RelationIsValid(r)) elog(ERROR, "could not open relation with OID %u", relationId);

	
	Assert(lockmode != NoLock || IsBootstrapProcessingMode() || CheckRelationLockedByMe(r, AccessShareLock, true));

	
	if (RelationUsesLocalBuffers(r)) MyXactFlags |= XACT_FLAGS_ACCESSEDTEMPNAMESPACE;

	pgstat_initstats(r);

	return r;
}