postgresql xact（即clog）

xact就是事务，只不过有些地方用xact，有些地方用trans，有些用transaction。

从内部看，事务状态分为：

/*
 * transaction states - transaction state from server perspective
 */
typedef enum TransState
{
 TRANS_DEFAULT,    /* idle */
 TRANS_START,    /* transaction starting */
 TRANS_INPROGRESS,    /* inside a valid transaction */
 TRANS_COMMIT,     /* commit in progress */
 TRANS_ABORT,    /* abort in progress */
 TRANS_PREPARE     /* prepare in progress */
} TransState;

从用户角度看，事务状态分为：

/*
 *    transaction block states - transaction state of client queries
 *
 * Note: the subtransaction states are used only for non-topmost
 * transactions; the others appear only in the topmost transaction.
 */
typedef enum TBlockState
{
    /* not-in-transaction-block states */
    TBLOCK_DEFAULT,                /* idle */
    TBLOCK_STARTED,                /* running single-query transaction */

    /* transaction block states */
    TBLOCK_BEGIN,                /* starting transaction block */
    TBLOCK_INPROGRESS,            /* live transaction */
    TBLOCK_IMPLICIT_INPROGRESS, /* live transaction after implicit BEGIN */
    TBLOCK_PARALLEL_INPROGRESS, /* live transaction inside parallel worker */
    TBLOCK_END,                    /* COMMIT received */
    TBLOCK_ABORT,                /* failed xact, awaiting ROLLBACK */
    TBLOCK_ABORT_END,            /* failed xact, ROLLBACK received */
    TBLOCK_ABORT_PENDING,        /* live xact, ROLLBACK received */
    TBLOCK_PREPARE,                /* live xact, PREPARE received */

    /* subtransaction states */
    TBLOCK_SUBBEGIN,            /* starting a subtransaction */
    TBLOCK_SUBINPROGRESS,        /* live subtransaction */
    TBLOCK_SUBRELEASE,            /* RELEASE received */
    TBLOCK_SUBCOMMIT,            /* COMMIT received while TBLOCK_SUBINPROGRESS */
    TBLOCK_SUBABORT,            /* failed subxact, awaiting ROLLBACK */
    TBLOCK_SUBABORT_END,        /* failed subxact, ROLLBACK received */
    TBLOCK_SUBABORT_PENDING,    /* live subxact, ROLLBACK received */
    TBLOCK_SUBRESTART,            /* live subxact, ROLLBACK TO received */
    TBLOCK_SUBABORT_RESTART        /* failed subxact, ROLLBACK TO received */
} TBlockState;

默认都是空闲，由全局变量TopTransactionStateData定义：

/*
 * CurrentTransactionState always points to the current transaction state
 * block.  It will point to TopTransactionStateData when not in a
 * transaction at all, or when in a top-level transaction.
 */
static TransactionStateData TopTransactionStateData = {
    .state = TRANS_DEFAULT,
    .blockState = TBLOCK_DEFAULT,
};

static TransactionState CurrentTransactionState = &TopTransactionStateData;

PS：在PG中，事务是支持超时的（还有其他很多逻辑比如死锁检测）。超时包括下列类型：

/*
 * Identifiers for timeout reasons.  Note that in case multiple timeouts
 * trigger at the same time, they are serviced in the order of this enum.
 */
typedef enum TimeoutId
{
    /* Predefined timeout reasons */
    STARTUP_PACKET_TIMEOUT,
    DEADLOCK_TIMEOUT,
    LOCK_TIMEOUT,
    STATEMENT_TIMEOUT,
    STANDBY_DEADLOCK_TIMEOUT,
    STANDBY_TIMEOUT,
    STANDBY_LOCK_TIMEOUT,
    IDLE_IN_TRANSACTION_SESSION_TIMEOUT,
    /* First user-definable timeout reason */
    USER_TIMEOUT,
    /* Maximum number of timeout reasons */
    MAX_TIMEOUTS = 16
} TimeoutId;

好奇一下，超时是什么机制做的？是基于SIGALRM信号触发的，超时管理实现在src/backend/utils/misc/timeout.c中，超时后是可以回调函数的。前3个事务ID保留：

#define InvalidTransactionId   ((TransactionId) 0)
#define BootstrapTransactionId  ((TransactionId) 1)
#define FrozenTransactionId    ((TransactionId) 2)
#define FirstNormalTransactionId ((TransactionId) 3)
#define MaxTransactionId   ((TransactionId) 0xFFFFFFFF)

事务ID包括：

LocalTransactionId：进程本地计数器生成，主要是为了优化。

VirtualTransactionId：分为两部分，前半部分为BackendId，后半部分为LocalTransactionId。

TransactionId/MultiXactId：主要是非专用区别场景使用，就是不需要明显区分，不需要特别的区分上下文。

SubTransactionId：子事务ID。