PostgreSQL存储页数据结构

PostgreSQL页存储

为了节省存储空间，HeapTupleFields和DatumTupleFields在元组头中分享同一个存储结构。但是两个分别是什么？如何实现重叠？何时替换掉？还不是很清楚

typedef struct HeapTupleFields
{
	TransactionId t_xmin;		/* inserting xact ID */
	TransactionId t_xmax;		/* deleting or locking xact ID */

	union
	{
		CommandId	t_cid;	/* inserting or deleting command ID, or both */
		TransactionId t_xvac;	/* old-style VACUUM FULL xact ID */
	}t_field3;
} HeapTupleFields;

typedef struct DatumTupleFields
{
	int32		datum_len_;	/* varlena header (do not touch directly!) */

	int32		datum_typmod;	/* -1, or identifier of a record type */

	Oid		datum_typeid;	/* composite type OID, or RECORDOID */

	/*
	 * Note: field ordering is chosen with thought that Oid might someday
	 * widen to 64 bits.
	 */
} DatumTupleFields;

struct HeapTupleHeaderData
{
	union
	{
		HeapTupleFields t_heap;
		DatumTupleFields t_datum;
	}t_choice;

	ItemPointerData t_ctid;		/* 指向当前元组的最新位置的偏移量和长度 */
	uint16		t_infomask2;	/* number of attributes + various flags */
	uint16		t_infomask;		/* various flag bits, see below */

	uint8		t_hoff;			/* sizeof header incl. bitmap, padding */

	/* ^ - 23 bytes - ^ */

	bits8		t_bits[FLEXIBLE_ARRAY_MEMBER];	/* bitmap of NULLs */

	/* MORE DATA FOLLOWS AT END OF STRUCT */
};

其中t_infomask的取值有以下几种：

#define HEAP_HASNULL		0x0001	/* has null attribute(s) */
#define HEAP_HASVARWIDTH	0x0002	/* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL	0x0004	/* has external stored attribute(s) */
#define HEAP_HASOID		0x0008	/* has an object-id field */

#define HEAP_XMAX_KEYSHR_LOCK	0x0010	/* xmax is a key-shared locker */
#define HEAP_COMBOCID		0x0020	/* t_cid is a combo cid 当元组在同一个事务中插入然后删除时使用*/
#define HEAP_XMAX_EXCL_LOCK	0x0040	/* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY	0x0080	/* xmax, if valid, is only a locker */

#define HEAP_XMIN_COMMITTED	0x0100	/* t_xmin committed */
#define HEAP_XMIN_INVALID	0x0200	/* t_xmin invalid/aborted */
#define HEAP_XMAX_COMMITTED	0x0400	/* t_xmax committed */
#define HEAP_XMAX_INVALID	0x0800	/* t_xmax invalid/aborted */

#define HEAP_XMAX_IS_MULTI	0x1000	/* t_xmax is a MultiXactId */
#define HEAP_UPDATED		0x2000	/* this is UPDATEd version of row */
#define HEAP_MOVED_OFF		0x4000	/* moved to another place by pre-9.0 VACUUM FULL; kept for binary upgrade support */
#define HEAP_MOVED_IN		0x8000	/* moved from another place by pre-9.0 VACUUM FULL; kept for binary upgrade support */

#define HEAP_XMAX_SHR_LOCK    (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_LOCK_MASK        (HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)

#define HEAP_XMIN_FROZEN        (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
#define HEAP_MOVED             (HEAP_MOVED_OFF | HEAP_MOVED_IN)

#define HEAP_XMAX_IS_LOCKED_ONLY(infomask) 
	(((infomask) & HEAP_XMAX_LOCK_ONLY) || 
	 (((infomask) & (HEAP_XMAX_IS_MULTI | HEAP_LOCK_MASK)) == HEAP_XMAX_EXCL_LOCK))

/*
 * information stored in t_infomask2:
 */
#define HEAP_NATTS_MASK			0x07FF	/* 11 bits for number of attributes */
/* bits 0x1800 are available */
#define HEAP_KEYS_UPDATED		0x2000	/* tuple was updated and key cols
										 * modified, or tuple deleted */
#define HEAP_HOT_UPDATED		0x4000	/* tuple was HOT-updated */
#define HEAP_ONLY_TUPLE			0x8000	/* this is heap-only tuple */

#define HEAP2_XACT_MASK			0xE000	/* visibility-related bits */