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 |