是否可以在postgresql上使用NOTIFY而不触发预写日志中的提交？

Question

我一直通过 PostgreSQL 使用 socket.io https://socket.io/docs/v4/postgres-adapter/ （它使用 pg 节点模块和 pool.query ）。这已经实现了一些心跳。问题是 NOTIFY 每秒都会在预写日志中触发相当空的提交。

可以避免提交吗？

Answer 1

这很不幸，但我认为这是无法避免的。查看

PreCommit_Notify()

来源：

        /*
         * Make sure that we have an XID assigned to the current transaction.
         * GetCurrentTransactionId is cheap if we already have an XID, but not
         * so cheap if we don't, and we'd prefer not to do that work while
         * holding NotifyQueueLock.
         */
        (void) GetCurrentTransactionId();

        /*
         * Serialize writers by acquiring a special lock that we hold till
         * after commit.  This ensures that queue entries appear in commit
         * order, and in particular that there are never uncommitted queue
         * entries ahead of committed ones, so an uncommitted transaction
         * can't block delivery of deliverable notifications.
         *
         * We use a heavyweight lock so that it'll automatically be released
         * after either commit or abort.  This also allows deadlocks to be
         * detected, though really a deadlock shouldn't be possible here.
         *
         * The lock is on "database 0", which is pretty ugly but it doesn't
         * seem worth inventing a special locktag category just for this.
         * (Historical note: before PG 9.0, a similar lock on "database 0" was
         * used by the flatfiles mechanism.)
         */
        LockSharedObject(DatabaseRelationId, InvalidOid, 0,
                         AccessExclusiveLock);

锁需要事务 ID，这两个会导致您观察到的 WAL 刷新。

我不知道你在使用

LISTEN

和

NOTIFY

做什么，但也许其他进程间通信方式可以解决这个问题，例如 咨询锁。