对PostgreSQL中 pg_各表的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))));
    else if (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：

pg_default_acl	826
pg_pltemplate	1136
pg_tablespace	1213
pg_shdepend	1214
pg_type	1247
pg_attribute	1249
pg_proc	1255
pg_class	1259
pg_authid	1260
pg_auth_members	1261
pg_database	1262
pg_foreign_server	1417
pg_user_mapping	1418
pg_foreign_data_wrapper	2328
pg_shdescription	2396
pg_aggregate	2600
pg_am	2601
pg_amop	2602
pg_ampro	2603
pg_attrdef	2604
pg_cast	2605
pg_constraint	2606
pg_conversion	2607
pg_depend	2608
pg_description	2609
pg_index	2610
pg_inherits	2611
pg_language	2612
pg_largeobject	2613
pg_namespace	2615
pg_opclass	2616
pg_operator	2617
pg_rewrite	2618
pg_stastic	2619
pg_trigger	2620
pg_opfamily	2753
pg_db_role_setting	2964
pg_largeobject_metadata	2995
pg_extension	3079
pg_foreign_table	3118
pg_collation	3456
pg_enum	3501
pg_seclabel	3596
pg_ts_dict	3600
pg_ts_parser	3601
pg_ts_config	3602
pg_ts_config_map	3603
pg_ts_template	3764

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 < MAX_LOCKMODES);

    /* Get the lock before trying to open the relcache entry */
    if (lockmode != NoLock)
        LockRelationOid(relationId, lockmode);

    /* The relcache does all the real work... */
    r = RelationIdGetRelation(relationId);

    fprintf(stderr,"In relation_open ,the relNode is:%d....\n\n",r->rd_node.relNode);

    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：

system table name	RelationId	FileName
pg_default_acl	826	12642
pg_pltemplate	1136	12591
pg_tablespace	1213	12587
pg_shdepend	1214	12598
pg_type	1247	12442
pg_attribute	1249	12446
pg_proc	1255	12458
pg_class	1259	12465
pg_authid	1260	12450
pg_auth_members	1261	12594
pg_database	1262	12692
pg_foreign_server	1417	12635
pg_user_mapping	1418	12454
pg_foreign_data_wrapper	2328	12631
pg_shdescription	2396	12602
pg_aggregate	2600	12525
pg_am	2601	12505
pg_amop	2602	12509
pg_ampro	2603	12514
pg_attrdef	2604	12469
pg_cast	2605	12549
pg_constraint	2606	12476
pg_conversion	2607	12562
pg_depend	2608	12567
pg_description	2609	12543
pg_index	2610	12489
pg_inherits	2611	12485
pg_language	2612	12518
pg_largeobject	2613	12571
pg_namespace	2615	12558
pg_opclass	2616	12501
pg_operator	2617	12493
pg_rewrite	2618	12528
pg_stastic	2619	12436
pg_trigger	2620	12535
pg_opfamily	2753	12497
pg_db_role_setting	2964	12581
pg_largeobject_metadata	2995	12522
pg_extension	3079	12627
pg_foreign_table	3118	12639
pg_collation	3456	12652
pg_enum	3501	12553
pg_seclabel	3596	12646
pg_ts_dict	3600	12615
pg_ts_parser	3601	12619
pg_ts_config	3602	12608
pg_ts_config_map	3603	12612
pg_ts_template	3764	12623

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）：

system table name	RelationId	FileName
pg_pltemplate	1136	12591
pg_tablespace	1213	12587
pg_shdepend	1214	12598
pg_authid	1260	12450
pg_auth_members	1261	12594
pg_database	1262	12692
pg_shdescription	2396	12602
pg_db_role_setting	2964	12581