| From: | "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Craig Ringer <craig(dot)ringer(at)2ndquadrant(dot)com>, Joe Conway <mail(at)joeconway(dot)com>, Mario Becroft <mb(at)true(dot)group>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Ilya Shkuratov <motr(dot)ilya(at)ya(dot)ru>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Gavin Flower <GavinFlower(at)archidevsys(dot)co(dot)nz>, Andrew Dunstan <andrew(dot)dunstan(at)2ndquadrant(dot)com>, Serge Rielau <serge(at)rielau(dot)com> |
| Subject: | Re: CTE inlining |
| Date: | 2017-05-09 19:51:58 |
| Message-ID: | CAKFQuwYpQ0XoyEWaxQtDY+soaSZhPoLr_SxFizgaEakfiT_=cw@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Tue, May 9, 2017 at 12:36 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Also, considering that this behavior has been there since 8.4,
> I think it's sheerest chutzpah to claim that changing the docs in
> v10 would materially reduce the backward-compatibility concerns
> for whatever we might do in v11.
>
No it won't, but those who are using 10 as their first version would
probably be happy if this was covered in a bit more depth. Even a comment
like "Unlike most other DBMS PostgreSQL presently executes the subquery in
the CTE in relative isolation. It is suggested that you document any
intentional usage of this optimization fence as a query planning hint so
that should the default change in the future you can update the query to
support whatever official syntax is implemented to retain this behavior.
We cannot really help those who have been using this since 8.4 and won't
read the relevant docs because they don't need to learn anything new; but
we can at least avoid subjecting newer customers. I'm not that strongly in
favor of it but it would be a nice touch - even if it won't change the
risk/reward calculation in any meaningful way.
David J.
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