PostgreSQL特性矩阵解析系列1之large objects

select pg_catalog.lo_create('521');#创建了一个oid为0521的large object。
test=# select * fr
om pg_largeobjec | lomowner t_metadata; #查询large object元数据表。
 oid | lomowner | lomacl 
-----+----------+--------
 521 |       10 |

test=# begin;
BEGIN
test=# select pg_catalog.lo_open('521',131072);
 lo_open 
---------
       0
(1 row)




test=# select pg_catalog.lowrite(0, '\\x000001000100101010000000000028010000160000002800000010000000200000000100040');
 lowrite 
---------
      77
(1 row)




test=# select pg_catalog.lo_close(0);
 lo_close 
----------
        0
(1 row)




test=# commit;
COMMIT




test=# select * from pg_largeobject;
 loid | pageno |                                                                             data                                     
                                        
------+--------+----------------------------------------------------------------------------------------------------------------------
----------------------------------------
  521 |      0 | \x5c78303030303031303030313030313031303130303030303030303030303238303130303030313630303030303032383030303030303130303
030303030323030303030303030313030303430
(1 row)

test=# create table lo_test(lo oid);#存储为oid
CREATE TABLE
test=# insert into lo_test(lo) values(521);
INSERT 0 1
test=# select * from lo_test;
 lo  
-----
 521

src/backend/catalog/pg_largeobject.c
/*
 * 创建一个具有给定 LO 标识符的大对象.
 *
 * 通过在pg_largeobject_metadata中插入一个条目(不含任何数据页)来创建一个新的大对象，这样该对象的大小就会显示为0。
 */
Oid LargeObjectCreate(Oid loid){
Relation  pg_lo_meta;
  Relation  pg_largeobject;
  ScanKeyData skey[1];
  SysScanDesc scan;
  HeapTuple  tuple;




  pg_lo_meta = table_open(LargeObjectMetadataRelationId, # RelationId=2995 此值为固定值,该值也为 pg_largeobject_metadata的relfilenode
              RowExclusiveLock);
              


  pg_largeobject = table_open(LargeObjectRelationId,
                RowExclusiveLock);
    ... ... ...
}




/*
 *删除具有给定LO标识符的大对象。数据页和元数据都必须删除。
 */
void LargeObjectDrop(Oid loid){...}




/*
 * 对于大对象元数据，我们不使用系统缓存，因为害怕使用过多的本地内存。
 * 此函数始终使用最新快照扫描系统目录，因此在以只读模式打开大对象时不应使用它（因为以只读模式打开的大对象应该相对于 *调用者的快照，而在读写模式下它们是相对于当前快照的。
 */
bool LargeObjectExists(Oid loid){...}




src/backend/catalog/pg_largeobject.h
/* ----------------
 * CPP将此转换为typedef struct formdata pg largeObject
 * ----------------
 */
CATALOG(pg_largeobject,2613,LargeObjectRelationId) #2163该值也为 pg_largeobject的relfilenode
{
  Oid      loid;      /* Identifier of large object */
  int32    pageno;      /* Page number (starting from 0) */




  /* data has variable length, but we allow direct access; see inv_api.c */
  bytea    data BKI_FORCE_NOT_NULL;  /* Data for page (may be
                       * zero-length) */
} FormData_pg_largeobject;

src/backend/catalog/pg_largeobject_metadata.h
/* ----------------
 *cpp将此转换为typedef struct FormData_pg_largeobject_metadata
 * ----------------
 */
CATALOG(pg_largeobject_metadata,2995,LargeObjectMetadataRelationId)
{
  Oid      oid;      /* oid */




  Oid      lomowner;    /* OID of the largeobject owner */




#ifdef CATALOG_VARLEN      /* variable-length fields start here */
  aclitem    lomacl[1];    /* access permissions */
#endif
} FormData_pg_largeobject_metadata;




/* ----------------
 *  Form_pg_largeobject_metadata对应一个指向pg_largeobject_metadata关系格式的元组的指针。
 * ----------------
 */
typedef FormData_pg_largeobject_metadata *Form_pg_largeobject_metadata;nclude/catalog/pg_largeobject_metadata.h