Date: Tue, 30 Jun 2020 14:09:22 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Fujii Masao <masao.fujii@oss.nttdata.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>,
	Michael Paquier <michael@paquier.xyz>,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: min_safe_lsn column in pg_replication_slots view
Message-ID: <20200630180922.GA13453@alvherre.pgsql>
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On 2020-Jun-30, Fujii Masao wrote:

> Sorry this is not true. That distance can be calculated without those operators.
> For example,
> 
> SELECT restart_lsn - pg_current_wal_lsn() + (SELECT setting::numeric * 1024 * 1024 FROM pg_settings WHERE name = 'max_slot_wal_keep_size') distance FROM pg_replication_slots;
> 
> If the calculated distance is small or negative value, which means that
> we may lose some required WAL files. So in this case it's worth considering
> to increase max_slot_wal_keep_size.

... OK, but you're forgetting wal_keep_segments.

> I still think it's better and more helpful to display something like
> that distance in pg_replication_slots rather than making each user
> calculate it...

Agreed.

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