Re: Explain Analyze (Rollback off) Suggestion

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: "David G(dot) Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Explain Analyze (Rollback off) Suggestion
Date: 2020-05-28 00:31:27
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Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I think the only way to make the effects of an EXPLAIN ANALYZE
> statement be automatically rolled back would be to wrap the entire
> operation in a subtransaction. While we could certainly implement
> that, it might have its own share of surprises; for example, it would
> consume an XID, leading to faster wraparound vacuums if you do it
> frequently.

Right, but it's just automating something that people now do by hand
(ie manually wrap the EXPLAIN in BEGIN/ROLLBACK) when that's what they
need. I think the idea of having an option to do it for you isn't bad.

I'm strongly against changing the very-longstanding default behavior of
EXPLAIN ANALYZE, though; the villagers at your doorstep will not be
bringing flowers. So this new option has to *not* default to on.

As far as the general topic of the thread goes, I like the idea of
controlling EXPLAIN options on the client side way better than inventing
statement-behavior-altering GUCs. We learned our lesson about that a
decade or two back; only those who don't remember propose new ones.

regards, tom lane

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