数据库内核月报 － 2017 / 10

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

	ItemPointerData t_ctid;		/* current TID of this or newer tuple (or a
								 * speculative insertion token) */

	/* Fields below here must match MinimalTupleData! */

	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 */
};

#define HEAP_HASNULL        0x0001    /* has null attribute(s) */
#define HEAP_HASVARWIDTH        0x0002    /* has variable-width attribute(s) 有可变参数 */
#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 */
#define HEAP_XMAX_EXCL_LOCK    0x0040    /* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY    0x0080    /* xmax, if valid, is only a locker */
/* xmax is a shared locker */
#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_COMMITTED    0x0100    /* t_xmin committed 即xmin已经提交*/
#define HEAP_XMIN_INVALID        0x0200    /* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN        (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
#define HEAP_XMAX_COMMITTED    0x0400    /* t_xmax committed即xmax已经提交*/
#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_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)
#define HEAP_XACT_MASK        0xFFF0    /* visibility-related bits */

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; /* VACUUM FULL xact ID */
}    t_field3;
} HeapTupleFields;

postgres=# create table test(id int);
CREATE TABLE
postgres=# insert into test values(1);
INSERT 0 1
postgres=# select ctid, xmin, xmax, cmin, cmax,id from test;
 ctid  | xmin | xmax | cmin | cmax | id
-------+------+------+------+------+----
 (0,1) | 1834 |    0 |    0 |    0 |  1
(1 row)

postgres=# update test set id=2;
UPDATE 1
postgres=# select ctid, xmin, xmax, cmin, cmax,id from test;
 ctid  | xmin | xmax | cmin | cmax | id
-------+------+------+------+------+----
 (0,2) | 1835 |    0 |    0 |    0 |  2
(1 row)

postgres=# select * from heap_page_items(get_raw_page('test',0));
 lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits | t_oid
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------+-------
  1 |   8160 |        1 |     28 |   1834 |   1835 |        0 | (0,2)  |       16385 |       1280 |     24 |        |
  2 |   8128 |        1 |     28 |   1835 |      0 |        0 | (0,2)  |       32769 |      10496 |     24 |        |

typedef struct SnapshotData
{
    SnapshotSatisfiesFunc satisfies;    /* tuple test function */
    TransactionId xmin;        /* all XID < xmin are visible to me */
    TransactionId xmax;        /* all XID >= xmax are invisible to me */
    TransactionId *xip;    //所有正在运行的事务的id列表
    uint32    xcnt;    /* # of xact ids in xip[]，正在运行的事务的计数 */
    TransactionId *subxip;           //进程中子事务的ID列表
    int32    subxcnt;        /* # of xact ids in subxip[]，进程中子事务的计数 */
    bool    suboverflowed;    /* has the subxip array overflowed? */
    bool    takenDuringRecovery;    /* recovery-shaped snapshot? */
    bool    copied;            /* false if it's a static snapshot */
    CommandId    curcid;    /* in my xact, CID < curcid are visible */
    uint32    speculativeToken;
    uint32    active_count;    /* refcount on ActiveSnapshot stack，在活动快照链表里的
*引用计数 */
    uint32    regd_count;    /* refcount on RegisteredSnapshots，在已注册的快照链表
*里的引用计数 */
    pairingheap_node ph_node;    /* link in the RegisteredSnapshots heap */
    TimestampTz  whenTaken;    /* timestamp when snapshot was taken */
    XLogRecPtr   lsn;        /* position in the WAL stream when taken */
} SnapshotData;