postgresqlのlarge object

//          ,pg_largeobject_metadata       ,     pg_largeobject



CATALOG(pg_largeobject_metadata,2995)
{
	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;

/*
 * Each "page" (tuple) of a large object can hold this much data
 *
 * We could set this as high as BLCKSZ less some overhead, but it seems
 * better to make it a smaller value, so that not as much space is used
 * up when a page-tuple is updated.  Note that the value is deliberately
 * chosen large enough to trigger the tuple toaster, so that we will
 * attempt to compress page tuples in-line.  (But they won't be moved off
 * unless the user creates a toast-table for pg_largeobject...)
 *
 * Also, it seems to be a smart move to make the page size be a power of 2,
 * since clients will often be written to send data in power-of-2 blocks.
 * This avoids unnecessary tuple updates caused by partial-page writes.
 */
#define LOBLKSIZE  (BLCKSZ / 4)

CATALOG(pg_largeobject,2613) BKI_WITHOUT_OIDS
{
	Oid			loid;			/* Identifier of large object */
	int4		pageno;			/* Page number (starting from 0) */
	/* data has variable length, but we allow direct access; see inv_api.c */
	bytea		data;			/* Data for page (may be zero-length) */
} FormData_pg_largeobject;

すべての大きいオブジェクトはすべてLOBLKSIZEの大きさに分解して、pg_を入れますLargeobjectではloidで区別し,各大オブジェクトが分割する各部分はpageno(メタグループ番号と呼ぶ方が合理的)で順序の関連付けを行い,
pagenoは、書き込むデータoffset/LOBLKSIZEで計算されます
pgの大きいオブジェクトの実現は比較的に簡単で、大きいデータ量の使用に適していない、システムのボトルネックになる.