Re: "SELECT ... FROM DUAL" is not quite as silly as it appears

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: "SELECT ... FROM DUAL" is not quite as silly as it appears
Date: 2018-10-15 03:25:09
Message-ID: 9565.1539573909@sss.pgh.pa.us
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I wrote:
> * There's a hack in nodeResult.c to prevent the executor from crashing
> on a whole-row Var for an RTE_RESULT RTE, which is something that the
> planner will create in SELECT FOR UPDATE cases, because it thinks it
> needs to provide a ROW_MARK_COPY image of the RTE's output. We might
> be able to get rid of that if we could teach the planner that it need
> not bother rowmarking RESULT RTEs, but that seemed like it would be
> really messy. (At the point where the decision is made, we don't know
> whether a subquery might end up as just a RESULT, or indeed vanish
> entirely.) Since I couldn't measure any reproducible penalty from
> having the extra setup cost for a Result plan, I left it like this.

Well, I'd barely sent this when I realized that there was a better way.
The nodeResult.c hack predates my decision to postpone cleaning up
RTE_RESULT RTEs till near the end of the preprocessing phase, and
given that code, it is easy to get rid of rowmarking RESULT RTEs ...
in fact, the code was doing it already, except in the edge case of
a SELECT with only a RESULT RTE. So here's a version that does not
touch nodeResult.c.

regards, tom lane

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