Received: from malur.postgresql.org ([217.196.149.56]) by arkaria.postgresql.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1stVjO-00H03e-Lp for pgsql-general@arkaria.postgresql.org; Wed, 25 Sep 2024 17:23:15 +0000 Received: from localhost ([127.0.0.1] helo=malur.postgresql.org) by malur.postgresql.org with esmtp (Exim 4.94.2) (envelope-from ) id 1stVjN-00BUWr-AC for pgsql-general@arkaria.postgresql.org; Wed, 25 Sep 2024 17:23:13 +0000 Received: from makus.postgresql.org ([2001:4800:3e1:1::229]) by malur.postgresql.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1stVjM-00BUWi-UB for pgsql-general@lists.postgresql.org; Wed, 25 Sep 2024 17:23:12 +0000 Received: from mail-lj1-x232.google.com ([2a00:1450:4864:20::232]) by makus.postgresql.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.94.2) (envelope-from ) id 1stVjK-0011eS-Ep for pgsql-general@lists.postgresql.org; Wed, 25 Sep 2024 17:23:11 +0000 Received: by mail-lj1-x232.google.com with SMTP id 38308e7fff4ca-2f77fe7ccc4so739381fa.2 for ; Wed, 25 Sep 2024 10:23:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1727284989; x=1727889789; darn=lists.postgresql.org; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=1HePAZpHd/nw3S7xv2U0qJ9Ff4v5ie+e99wlQD4c2/0=; b=OwxGTiTAlmAEIZjRs7IdyUNTmKjjUunPT2b9YCAy5K7lYgHsar+S3p7+1XiobB+yRp cLEvFuKWl/xx8iMDB1OiDzdxZU/3Nl29qVy+vy+T2R1wqXCyFa5XwtjdSRSPrSOv1StO SJSVLXb85n+4yNeIGAbmN5kBscGusPF3MLyb5F7Gj93WwUmj2FVlwYArY6SpqgoW9lvT baKzfV4ZiNpzFbODwspFwBtLle5oV+XcDVFxbgTijFCwaZ5Ix0hkJ7BnHkGEwK0URtYb aR4j8E2I57xpGbPkFtTwMst7CnUpi7Fwk0L/WsOpo16eDTUv6awh/J5e/5zv0TMB6peL rdWg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1727284989; x=1727889789; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=1HePAZpHd/nw3S7xv2U0qJ9Ff4v5ie+e99wlQD4c2/0=; b=Pu9x+mfSvOzHckX4FbfPgV+ApjLWd+7ysZOBZsUbEHvroIDFDJOG8i2vxpZvE4uSCh o4Chfwlw7yA2P3sQbg24FOrUcZ1ync9q9SYBmeVjDgVdFfJ1P/xOqJbui6enRbrLgUNy u09oCSLqAhLnW7xTh/VW+oqsvwFVMzEmUZHLo1AT740tcACFodyr3N408pIUTNaLlnl+ yF4Xik+/IIxISLRXziTNVz7Ug3UDKNZe/a6qWlM6jZwY2p06O8mwQz/+oPwEs1QcmBuP sJmrS6npRPtzLZOc+eiksxrs4xSUeSNoxxMCgW8U8wGXDJcZL60iMPL7kIVVgK9ZCq7j BN0Q== X-Forwarded-Encrypted: i=1; AJvYcCU9HIKJaB1GyWgYGAGOg+kZQvaRAuOh66mxTS9sulNNyRbobidDdsaQ7VY7T7wIQx1I1S1xgC/XLRnPCKmV@lists.postgresql.org X-Gm-Message-State: AOJu0YxyvITBI02FIn67gEg+6MUhY7GdYX7fNJs9ZVWf49Kda4osSYEF PE4szyjRwnZMkNlBzEF7JMr4ahzLYnanNL31zSDBTTkpn3zNsLtnssdOpz+GntXOGeB1H0Fc1XQ WT4opF1NP9dpyqcxo/w2mX6M1izM= X-Google-Smtp-Source: AGHT+IFvPzNk77j9200uBVsJ1txdId1UcGETsKEo0wJXv73k54dRvvWZ8ZP8zMAGGBNGMJ8n5xgTgK/RvtbnFmpL+Iw= X-Received: by 2002:a2e:bc05:0:b0:2f7:5d53:7196 with SMTP id 38308e7fff4ca-2f915fc2607mr28392431fa.4.1727284988443; Wed, 25 Sep 2024 10:23:08 -0700 (PDT) MIME-Version: 1.0 References: <9CEBFAC7-4372-4FF0-8124-FFFE834B03C6@gmail.com> <3346993.1727188126@sss.pgh.pa.us> In-Reply-To: <3346993.1727188126@sss.pgh.pa.us> From: Greg Sabino Mullane Date: Wed, 25 Sep 2024 13:22:31 -0400 Message-ID: Subject: Re: Repeatable Read Isolation Level "transaction start time" To: Tom Lane Cc: Wizard Brony , pgsql-general@lists.postgresql.org Content-Type: multipart/alternative; boundary="0000000000002984df0622f4e0a3" List-Id: List-Help: List-Subscribe: List-Post: List-Owner: List-Archive: Archived-At: Precedence: bulk --0000000000002984df0622f4e0a3 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, Sep 24, 2024 at 10:28=E2=80=AFAM Tom Lane wrote= : > It's even looser than that, really: it's the first statement that > requires an MVCC snapshot. Hm....so why does "SELECT 1;" work as a transaction start marker then, as opposed to "SHOW work_mem;", which does not? Do we simply consider anything with a SELECT as needing a snapshot? Cheers, Greg --0000000000002984df0622f4e0a3 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On Tue, Sep 24, 2024 at 10:28=E2=80=AFAM = Tom Lane <tgl@sss.pgh.pa.us>= wrote:
It's even looser than that, really: it's the fir= st statement that
requires an MVCC snapshot.

Hm....so why doe= s "SELECT 1;" work as a transaction start marker then, as opposed= to "SHOW work_mem;", which does not? Do we simply consider anyth= ing with a SELECT as needing a snapshot?

Cheers,
Greg

--0000000000002984df0622f4e0a3--