PostgreSQL中 pg_各表RelationId的认识

读取普通的table或者系统表，都会调用heap_open函数：

/* ----------------
 *        heap_open - open a heap relation by relation OID
 *
 *        This is essentially relation_open plus check that the relation
 *        is not an index nor a composite type.  (The caller should also
 *        check that it's not a view or foreign table before assuming it has
 *        storage.)
 * ----------------
 */
Relation
heap_open(Oid relationId, LOCKMODE lockmode)
{
    //fprintf(stderr,"++++++++++++++++++++ In heap_open start by process %d....relationId is:%d\n",
getpid(),relationId);
    Relation    r;

    r = relation_open(relationId, lockmode);

    if (r->rd_rel->relkind == RELKIND_INDEX)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is an index",
                        RelationGetRelationName(r))));
    elseif (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is a composite type",
                        RelationGetRelationName(r))));

    //fprintf(stderr,"++++++++++++++++++++ In heap_open end by process %d\n\n",getpid());

    return r;
}

对于普通表而言，RelationId就是在base目录下的某个子目录里面的文件名。

但是对于系统表而言，则不同。 比如 pg_tablespace 的RelationId为 1213（这已经写死在PostgreSQL源代码中）， 但是其对应的文件的名称为 12587（对应global/12587文件）。

经过一番测试，发现其对应关系如下：

然后，我还可以进一步，观察 ，把上述表格补充完整：

/* ----------------
 *        relation_open - open any relation by relation OID
 *
 *        If lockmode is not "NoLock", the specified kind of lock is
 *        obtained on the relation.  (Generally, NoLock should only be
 *        used if the caller knows it has some appropriate lock on the
 *        relation already.)
 *
 *        An error is raised if the relation does not exist.
 *
 *        NB: a "relation" is anything with a pg_class entry.  The caller is
 *        expected to check whether the relkind is something it can handle.
 * ----------------
 */
Relation
relation_open(Oid relationId, LOCKMODE lockmode)
{

    fprintf(stderr,"___________________ In relation_open start by process %d\n",getpid());

    Relation    r;

    Assert(lockmode >= NoLock && lockmode );

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

    /* Make note that we've accessed a temporary relation */if (RelationUsesLocalBuffers(r))
        MyXactAccessedTempRel = true;

    pgstat_initstats(r);

    fprintf(stderr,"___________________ In relation_open end by process %d\n",getpid());

    return r;
}

加入了调试代码后，我可以看到，pg_tablespace 的 RelationId是 1213，而它的对应文件名是 12587。

下面，补充完整：

如果进一步查看，还可以发现：

只有如下几个系统表的对应文件位于 global目录，其余的系统表的对应文件则是base目录下的每个子目录中都有（一个子目录对应一个数据库）：