typedef uint32 TransactionId;  /* 事務(wù)號定義，32位無符號整數(shù) */
typedef struct HeapTupleFields
{
  TransactionId t_xmin;    /* 插入該元組的事務(wù)號 */
  TransactionId t_xmax;    /* 刪除或鎖定該元組的事務(wù)號 */

    /*** 其它屬性省略 ***/
} HeapTupleFields;

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

    /*** 其它屬性省略 ***/
};

// 無效事務(wù)號
#define InvalidTransactionId    ((TransactionId) 0)
// 引導(dǎo)事務(wù)號，在數(shù)據(jù)庫初始化過程（BKI執(zhí)行）中使用
#define BootstrapTransactionId    ((TransactionId) 1)
// 凍結(jié)事務(wù)號用于表示非常陳舊的元組，它們比所有正常事務(wù)號都要早（也就是可見）
#define FrozenTransactionId      ((TransactionId) 2)
// 第一個正常事務(wù)號
#define FirstNormalTransactionId  ((TransactionId) 3)
// 把 FullTransactionId 的低32位作為無符號整數(shù)生成 xid
#define XidFromFullTransactionId(x)    ((uint32) (x).value)

static inline void
FullTransactionIdAdvance(FullTransactionId *dest)
{
  dest->value++;
  while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
    dest->value++;
}

FullTransactionId
GetNewTransactionId(bool isSubXact)
{
    /*** 省略 ***/
  full_xid = ShmemVariableCache->nextFullXid;
  xid = XidFromFullTransactionId(full_xid);
    /*** 省略 ***/
  FullTransactionIdAdvance(&ShmemVariableCache->nextFullXid);
    /*** 省略 ***
  return full_xid;
}

static void
AssignTransactionId(TransactionState s)
{
    /*** 省略 ***/
  s->fullTransactionId = GetNewTransactionId(isSubXact);
  if (!isSubXact)
    XactTopFullTransactionId = s->fullTransactionId;
    /*** 省略 ***/
}

TransactionId
GetTopTransactionId(void)
{
  if (!FullTransactionIdIsValid(XactTopFullTransactionId))
    AssignTransactionId(&TopTransactionStateData);
  return XidFromFullTransactionId(XactTopFullTransactionId);
}

/*
 * TransactionIdPrecedes --- is id1 logically < id2?
 */
bool
TransactionIdPrecedes(TransactionId id1, TransactionId id2)
{
  /*
   * If either ID is a permanent XID then we can just do unsigned
   * comparison.  If both are normal, do a modulo-2^32 comparison.
   */
  int32    diff;

  if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
    return (id1 < id2);

  diff = (int32) (id1 - id2);
  return (diff < 0);
}

# 查看每個庫的年齡
SELECT datname, age(datfrozenxid) FROM pg_database;

# 1個庫每個表的年齡排序
SELECT c.oid::regclass as table_name, greatest(age(c.relfrozenxid),age(t.relfrozenxid)) as age FROM pg_class c LEFT JOIN pg_class t ON c.reltoastrelid = t.oid WHERE c.relkind IN ('r', 'm') order by age desc; 

# 查看1個表的年齡
select oid::regclass,age(relfrozenxid) from pg_class where oid='schema名稱.表名稱'::regclass::oid;

vacuum freeze 表名;

vacuumdb -d 庫名 --freeze --jobs=30 -h 連接串 -p 端口號 -U 庫Owner

void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid, Oid oldest_datoid)
{
  TransactionId xidVacLimit;
  TransactionId xidWarnLimit;
  TransactionId xidStopLimit;
  TransactionId xidWrapLimit;
  TransactionId curXid;

  Assert(TransactionIdIsNormal(oldest_datfrozenxid));

  /*
     * xidWrapLimit = 最老的事務(wù)號 + 0x7FFFFFFF，當前事務(wù)號一旦到達xidWrapLimit將發(fā)生回卷
   */
  xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
  if (xidWrapLimit < FirstNormalTransactionId)
    xidWrapLimit += FirstNormalTransactionId;

  /*
     * 一旦當前事務(wù)號到達xidStopLimit，實例將不可寫入，保留 1000000 的xid用于vacuum
     * 每 vacuum 一張表需要占用一個xid
   */
  xidStopLimit = xidWrapLimit - 1000000;
  if (xidStopLimit < FirstNormalTransactionId)
    xidStopLimit -= FirstNormalTransactionId;

  /*
     * 一旦當前事務(wù)號到達xidWarnLimit，將不停地收到
     * WARNING:  database "xxxx" must be vacuumed within 2740112 transactions
   */
  xidWarnLimit = xidStopLimit - 10000000;
  if (xidWarnLimit < FirstNormalTransactionId)
    xidWarnLimit -= FirstNormalTransactionId;

  /*
     * 一旦當前事務(wù)號到達xidVacLimit將觸發(fā)force autovacuums
   */
  xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
  if (xidVacLimit < FirstNormalTransactionId)
    xidVacLimit += FirstNormalTransactionId;

  /* Grab lock for just long enough to set the new limit values */
  LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
  ShmemVariableCache->oldestXid = oldest_datfrozenxid;
  ShmemVariableCache->xidVacLimit = xidVacLimit;
  ShmemVariableCache->xidWarnLimit = xidWarnLimit;
  ShmemVariableCache->xidStopLimit = xidStopLimit;
  ShmemVariableCache->xidWrapLimit = xidWrapLimit;
  ShmemVariableCache->oldestXidDB = oldest_datoid;
  curXid = XidFromFullTransactionId(ShmemVariableCache->nextFullXid);
  LWLockRelease(XidGenLock);

  /* Log the info */
  ereport(DEBUG1,
      (errmsg("transaction ID wrap limit is %u, limited by database with OID %u",
          xidWrapLimit, oldest_datoid)));

  /*
     * 如果 當前事務(wù)號>=最老事務(wù)號+autovacuum_freeze_max_age
     * 觸發(fā) autovacuum 對年齡最老的數(shù)據(jù)庫進行清理，如果有多個數(shù)據(jù)庫達到要求，按年齡最老的順序依次清理
   * 通過設(shè)置標志位標記當前 autovacuum 結(jié)束之后再來一次 autovacuum
     */
  if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
    IsUnderPostmaster && !InRecovery)
    SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);

  /* Give an immediate warning if past the wrap warn point */
  if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit) && !InRecovery)
  {
    char     *oldest_datname;

    if (IsTransactionState())
      oldest_datname = get_database_name(oldest_datoid);
    else
      oldest_datname = NULL;

    if (oldest_datname)
      ereport(WARNING,
          (errmsg("database \"%s\" must be vacuumed within %u transactions",
              oldest_datname,
              xidWrapLimit - curXid),
           errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
               "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
    else
      ereport(WARNING,
          (errmsg("database with OID %u must be vacuumed within %u transactions",
              oldest_datoid,
              xidWrapLimit - curXid),
           errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.\n"
               "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
  }
}

bool
TransactionIdFollowsOrEquals(TransactionId id1, TransactionId id2)
{
  int32    diff;
  if (!TransactionIdIsNormal(id1) || !TransactionIdIsNormal(id2))
    return (id1 >= id2);

  diff = (int32) (id1 - id2);
  return (diff >= 0);
}

FullTransactionId
GetNewTransactionId(bool isSubXact)
{
    /*** 省略 ***/
  full_xid = ShmemVariableCache->nextFullXid;
  xid = XidFromFullTransactionId(full_xid);

  if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
  {
    TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
    TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
    TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
    Oid      oldest_datoid = ShmemVariableCache->oldestXidDB;

        /*** 省略 ***/
    if (IsUnderPostmaster &&
      TransactionIdFollowsOrEquals(xid, xidStopLimit))
    {
      char     *oldest_datname = get_database_name(oldest_datoid);

      /* complain even if that DB has disappeared */
      if (oldest_datname)
        ereport(ERROR,
            (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
             errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
                oldest_datname),
             errhint("Stop the postmaster and vacuum that database in single-user mode.\n"
                 "You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
            /*** 省略 ***/
    }
        /*** 省略 ***/
  }
    /*** 省略 ***/
}

# 查看每個庫的年齡
SELECT datname, age(datfrozenxid) FROM pg_database;

# 1個庫每個表的年齡排序
SELECT c.oid::regclass as table_name, greatest(age(c.relfrozenxid),age(t.relfrozenxid)) as age FROM pg_class c LEFT JOIN pg_class t ON c.reltoastrelid = t.oid WHERE c.relkind IN ('r', 'm') order by age desc; 

# 查看1個表的年齡
select oid::regclass,age(relfrozenxid) from pg_class where oid='schema名稱.表名稱'::regclass::oid;

# 對指定數(shù)據(jù)庫中年齡最大的前 50 張表進行 vacuum freeze

for cmd in `psql -U用戶名 -p端口號 -h連接串 -d數(shù)據(jù)庫名 -c "SELECT 'vacuum freeze '||c.oid::regclass||';' as vacuum_cmd FROM pg_class c LEFT JOIN pg_class t ON c.reltoastrelid = t.oid WHERE c.relkind IN ('r', 'm') order by greatest(age(c.relfrozenxid),age(t.relfrozenxid)) desc offset 50 limit 50;" | grep -v vacuum_cmd  | grep -v row | grep vacuum`; do
    psql -U用戶名 -p端口號 -h連接串 -d數(shù)據(jù)庫名 -c "$cmd"
done

from multiprocessing import Pool
import psycopg2

args = dict(host='pgm-bp10xxxx.pg.rds.aliyuncs.com', port=5432, dbname='數(shù)據(jù)庫名',
            user='用戶名', password='密碼')

def vacuum_handler(sql):
    sql_str = "SELECT c.oid::regclass as table_name, greatest(age(c.relfrozenxid),age(t.relfrozenxid)) as age FROM pg_class c LEFT JOIN pg_class t ON c.reltoastrelid = t.oid WHERE c.relkind IN ('r', 'm') order by age desc limit 10; "
    try:
        conn = psycopg2.connect(**args)
        cur = conn.cursor()
        cur.execute(sql)
        conn.commit()
        cur = conn.cursor()
        cur.execute(sql_str)
        print cur.fetchall()
        conn.close()
    except Exception as e:
        print str(e)

# 對指定數(shù)據(jù)庫中年齡最大的前 1000 張表進行 vacuum freeze，32 個進程并發(fā)執(zhí)行
def multi_vacuum():
    pool = Pool(processes=32)
    sql_str = "SELECT 'vacuum freeze '||c.oid::regclass||';' as vacuum_cmd FROM pg_class c LEFT JOIN pg_class t ON c.reltoastrelid = t.oid WHERE c.relkind IN ('r', 'm') order by greatest(age(c.relfrozenxid),age(t.relfrozenxid)) desc limit 1000;";
    try:
        conn = psycopg2.connect(**args)
        cur = conn.cursor()
        cur.execute(sql_str)
        rows = cur.fetchall()
        for row in rows:
            cmd = row['vacuum_cmd']
            pool.apply_async(vacuum_handler, (cmd, ))
        conn.close()
        pool.close()
        pool.join()
    except Exception as e:
        print str(e)


multi_vacuum()