From: Andy Fan <zhihuifan1213@163.com>
To: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Subject: FullTransactionIdAdvance question
cc: Andres Freund <andres@anarazel.de>
Date: Fri, 20 Sep 2024 17:38:40 +0800
Message-ID: <87frpu4qcf.fsf@163.com>
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Hi,

static inline void
FullTransactionIdAdvance(FullTransactionId *dest)
{
	dest->value++;

	/* see FullTransactionIdAdvance() */
	if (FullTransactionIdPrecedes(*dest, FirstNormalFullTransactionId))
		return;

	while (XidFromFullTransactionId(*dest) < FirstNormalTransactionId)
		dest->value++;
}

I understand this functiona as: 'dest->value++' increases the epoch when
necessary and we don't want use the TransactionId which is smaller than
FirstNormalTransactionId. But what is the point of the below code:

/* see FullTransactionIdAdvance() */
if (FullTransactionIdPrecedes(*dest, FirstNormalFullTransactionId))
	return;

It looks to me it will be never true(I added a 'Assert(false);' above
the return, make check-world pass). and if it is true somehow, retruning
a XID which is smaller than FirstNormalTransactionId looks strange as
well. IIUC, should we remove it to save a prediction on each
GetNewTransactionId call?   

-- 
Best Regards
Andy Fan