Date: Tue, 31 Mar 2020 14:18:36 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: jgdr@dalibo.com, andres@anarazel.de, michael@paquier.xyz,
	sawada.mshk@gmail.com, peter.eisentraut@2ndquadrant.com,
	pgsql-hackers@lists.postgresql.org, thomas.munro@enterprisedb.com,
	sk@zsrv.org, michael.paquier@gmail.com
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
Message-ID: <20200331171836.GA7973@alvherre.pgsql>
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On 2020-Mar-31, Alvaro Herrera wrote:

> I'm not sure if I explained my proposal clearly.  What if
> XLogGetLastRemovedSegno returning zero means that every segment is
> valid?  We don't need to scan pg_xlog at all.

I mean this:

XLogSegNo
FindOldestXLogFileSegNo(void)
{
	XLogSegNo segno = XLogGetLastRemovedSegno();

	/* this is the only special case we need to care about */
	if (segno == 0)
		return some-value;

	return segno + 1;
}

... and that point one can further note that a freshly initdb'd system
(no file has been removed) has "1" as the first file.  So when segno is
0, you can return 1 and all should be well.  That means you can reduce
the function to this:

XLogSegNo
FindOldestXLogFileSegNo(void)
{
	return XLogGetLastRemovedSegno() + 1;
}


The tests still pass with this coding.

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