Re: Improving connection scalability: GetSnapshotData()

From: Andres Freund <andres(at)anarazel(dot)de>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Peter Geoghegan <pg(at)bowt(dot)ie>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Jonathan Katz <jkatz(at)postgresql(dot)org>, Bruce Momjian <bruce(at)momjian(dot)us>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>
Subject: Re: Improving connection scalability: GetSnapshotData()
Date: 2020-04-07 19:30:55
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On 2020-04-07 14:51:52 -0400, Robert Haas wrote:
> On Tue, Apr 7, 2020 at 2:28 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Does that make some sense? Do you have a better suggestion for a name?
> I think it makes sense. I have two basic problems with the name. The
> first is that "on disk" doesn't seem to be a very clear way of
> describing what you're actually checking here, and it definitely
> doesn't refer to an existing concept which sophisticated hackers can
> be expected to understand. The second is that "may" is ambiguous in
> English: it can either mean that something is permissible ("Johnny,
> you may go to the bathroom") or that we do not have certain knowledge
> of it ("Johnny may be in the bathroom"). When it is followed by "be",
> it usually has the latter sense, although there are exceptions (e.g.
> "She may be discharged from the hospital today if she wishes, but we
> recommend that she stay for another day"). Consequently, I found that
> use of "may be" in this context wicked confusing.

Well, it *is* only a vague test :). It shouldn't ever have a false
positive, but there's plenty chance for false negatives (if wrapped
around far enough).

> So I suggest a name with "Is" or no verb, rather than one with
> "MayBe." And I suggest something else instead of "OnDisk," e.g.
> AssertTransactionIdIsInUsableRange() or
> TransactionIdIsInAllowableRange() or
> AssertTransactionIdWraparoundProtected(). I kind of like that last
> one, but YMMV.

Make sense - but they all seem to express a bit more certainty than I
think the test actually provides.

I explicitly did not want (and added a comment to that affect) have
something like TransactionIdIsInAllowableRange(), because there never
can be a safe use of its return value, as far as I can tell.

The "OnDisk" was intended to clarify that the range it verifies is
whether it'd be ok for the xid to have been found in a relation.


Andres Freund

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