PostgreSQL多版本的可见性判断

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;
详见：postgresql-12.2\src\include\access\htup_details.h

/*
 * Possible transaction statuses --- note that all-zeroes is the initial
 * state.
 *
 * A "subcommitted" transaction is a committed subtransaction whose parent
 * hasn't committed or aborted yet.
 */
typedef int XidStatus;

#define TRANSACTION_STATUS_IN_PROGRESS    0x00
#define TRANSACTION_STATUS_COMMITTED    0x01
#define TRANSACTION_STATUS_ABORTED      0x02
#define TRANSACTION_STATUS_SUB_COMMITTED  0x03
详见：postgresql-12.2\src\include\access\clog.h

#define HEAP_XMIN_COMMITTED    0x0100  /* t_xmin committed */
#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 */
#define HEAP_XMAX_INVALID    0x0800  /* t_xmax invalid/aborted */
#define HEAP_XMAX_IS_MULTI    0x1000  /* t_xmax is a MultiXactId */
详见：postgresql-12.2\src\include\access\htup_details.h

typedef struct SnapshotData
{
    ...
    TransactionId xmin;         /* all XID < xmin are visible to me */
    TransactionId xmax;         /* all XID >= xmax are invisible to me */
    ...
}
详见：\postgresql-12.2\postgresql-12.2\src\include\utils\snapshot.h

/* t_xmin status = ABORTED */
Rule 1: IF t_xmin status is 'ABORTED' THEN
            RETURN 'Invisible'
        END IF
        
/* t_xmin status = IN_PROGRESS */
        IF t_xmin status is 'IN_PROGRESS' THEN
            IF t_xmin = current_txid THEN
Rule 2:         IF t_xmax = INVALID THEN
                    RETURN 'Visible'
Rule 3:         ELSE  /* this tuple has been deleted or updated by the current transaction itself. */
                    RETURN 'Invisible'
                END IF
Rule 4:     ELSE   /* t_xmin ≠ current_txid */
                RETURN 'Invisible'
            END IF
         END IF


/* t_xmin status = COMMITTED */
         IF t_xmin status is 'COMMITTED' THEN
Rule 5:     IF t_xmin is active in the obtained transaction snapshot THEN
                RETURN 'Invisible'
Rule 6:     ELSE IF t_xmax = INVALID OR status of t_xmax is 'ABORTED' THEN
                RETURN 'Visible'
            ELSE IF t_xmax status is 'IN_PROGRESS' THEN
Rule 7:         IF t_xmax =  current_txid THEN
                    RETURN 'Invisible'
Rule 8:         ELSE  /* t_xmax ≠ current_txid */
                    RETURN 'Visible'
                END IF
            ELSE IF t_xmax status is 'COMMITTED' THEN
Rule 9:         IF t_xmax is active in the obtained transaction snapshot THEN
                    RETURN 'Visible'
Rule 10:        ELSE
                    RETURN 'Invisible'
                END IF
            END IF
        END IF
更多参考详见：http://www.interdb.jp/pg/pgsql05.html

Rule 1: If Status(t_xmin) = ABORTED => Invisible 
Rule 2: If Status(t_xmin) = IN_PROGRESS ∧ t_xmin = current_txid ∧ t_xmax = INVAILD => Visible
Rule 3: If Status(t_xmin) = IN_PROGRESS ∧ t_xmin = current_txid ∧ t_xmax ≠ INVAILD => Invisible
Rule 4: If Status(t_xmin) = IN_PROGRESS ∧ t_xmin ≠ current_txid => Invisible
Rule 5: If Status(t_xmin) = COMMITTED ∧ Snapshot(t_xmin) = active => Invisible 
Rule 6: If Status(t_xmin) = COMMITTED ∧ (t_xmax = INVALID ∨ Status(t_xmax) = ABORTED) => Visible  
Rule 7: If Status(t_xmin) = COMMITTED ∧ Status(t_xmax) = IN_PROGRESS ∧ t_xmax = current_txid => Invisible
Rule 8: If Status(t_xmin) = COMMITTED ∧ Status(t_xmax) = IN_PROGRESS ∧ t_xmax ≠ current_txid => Visible
Rule 9: If Status(t_xmin) = COMMITTED ∧ Status(t_xmax) = COMMITTED ∧ Snapshot(t_xmax) = active => Visible
Rule 10: If Status(t_xmin) = COMMITTED ∧ Status(t_xmax) = COMMITTED ∧ Snapshot(t_xmax) ≠ active => Invisible