Date: Thu, 2 May 2019 12:03:35 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@anarazel.de>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>,
	Masahiko Sawada <sawada.mshk@gmail.com>,
	Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>,
	"Bossart, Nathan" <bossartn@amazon.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: New vacuum option to do only freezing
Message-ID: <20190502160335.GA17452@alvherre.pgsql>
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On 2019-May-01, Andres Freund wrote:

> Alvaro, could we perhaps clean this up a bit? This is pretty confusing
> looking.  I think this probably could just be changed to
> 
>         bool            xmin_frozen = false;
> 
>         xid = HeapTupleHeaderGetXmin(tuple);
> 
>         if (xid == FrozenTransactionId)
>                 xmin_frozen = true;
>         else if (TransactionIdIsNormal(xid))
> 
> or somesuch.  There's no need to check for
> HeapTupleHeaderXminFrozen(tuple) etc, because HeapTupleHeaderGetXmin()
> already does so - and if it didn't, the issue Tom points out would be
> problematic.

Ah, yeah, that's simpler.  I would like to introduce a couple of very
minor changes to the proposed style, per the attached.

* don't initialize xmin_frozen at all; rather, only set its value to the
correct one when we have determined what it is.  Doing premature
initialization is what led to some of those old bugs, so I prefer not to
do it.

* Handle the BootstrapXid and InvalidXid cases explicitly, by setting
xmin_frozen to true when xmin is not normal.  After all, those XID
values do not need any freezing.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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 filename="0001-heap_prepare_freeze_tuple-Simplify-coding.patch"

From 1cca0581655614703b020facbbf8d64b442f95be Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Thu, 2 May 2019 11:50:18 -0400
Subject: [PATCH] heap_prepare_freeze_tuple: Simplify coding

---
 src/backend/access/heap/heapam.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index a05b6a07ad0..d6ceb8fd4a4 100644
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -6108,9 +6108,9 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 
 	/* Process xmin */
 	xid = HeapTupleHeaderGetXmin(tuple);
-	xmin_frozen = ((xid == FrozenTransactionId) ||
-				   HeapTupleHeaderXminFrozen(tuple));
-	if (TransactionIdIsNormal(xid))
+	if (!TransactionIdIsNormal(xid))
+		xmin_frozen = true;
+	else
 	{
 		if (TransactionIdPrecedes(xid, relfrozenxid))
 			ereport(ERROR,
@@ -6118,7 +6118,8 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 					 errmsg_internal("found xmin %u from before relfrozenxid %u",
 									 xid, relfrozenxid)));
 
-		if (TransactionIdPrecedes(xid, cutoff_xid))
+		xmin_frozen = TransactionIdPrecedes(xid, cutoff_xid);
+		if (xmin_frozen)
 		{
 			if (!TransactionIdDidCommit(xid))
 				ereport(ERROR,
@@ -6128,7 +6129,6 @@ heap_prepare_freeze_tuple(HeapTupleHeader tuple,
 
 			frz->t_infomask |= HEAP_XMIN_FROZEN;
 			changed = true;
-			xmin_frozen = true;
 		}
 	}
 
-- 
2.17.1


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