From: | Erik Brandsberg <erik(at)heimdalldata(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Martin Handsteiner <martin(dot)handsteiner(at)sibvisions(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, "pgsql-sql(at)lists(dot)postgresql(dot)org" <pgsql-sql(at)lists(dot)postgresql(dot)org> |
Subject: | Re: AW: Optimizer Hint, to ignore limit and offset in optimizer plan |
Date: | 2020-06-29 14:33:43 |
Message-ID: | CAFcck8H+4kWzYTc_TFzn1GF-1u39hU66RskSVbeWr12CGnzFaQ@mail.gmail.com |
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Lists: | pgsql-sql |
New request: If order by is not specified with an order by clause, allow
an option to be set that explicitly randomizes the results to break
everything that relies on the order. :) This would primarily be used for
QA work to find code that depends on an undefined order.
And waving hi at a fellow Yinzer!
On Mon, Jun 29, 2020 at 10:30 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Martin Handsteiner <martin(dot)handsteiner(at)sibvisions(dot)com> writes:
> > yes, this would exactly be the feature I was talking about.
> > enable_costlimit = true (default) | false
> > How high is the possibility, that this feature will find the way into
> the postgres db?
>
> Not very good, unless you can present a far more convincing use-case.
>
> If I understand your desire, it is that issuing the "same" query in
> different transactions would generate identical overall results
> despite varying the offset/limit so as to fetch different parts of
> that unchanging result.
>
> The problem with this is that whether changing the offset/limit
> changes the plan shape is just one of many reasons why you might
> not get a consistent result --- the most unavoidable being that
> other transactions might commit data changes.
>
> Moreover, you insist that you shouldn't have to use an ORDER BY
> to get these consistent results. Sorry, but SQL is *defined* to
> not produce consistent row ordering without ORDER BY. Changing
> that isn't a matter of some optimizer hint somewhere, it's a very
> fundamental thing in many places.
>
> I'd counsel taking another look at the suggestion made upthread
> to use a cursor WITH HOLD.
>
> regards, tom lane
>
>
>
--
*Erik Brandsberg*
erik(at)heimdalldata(dot)com
www.heimdalldata.com
+1 (866) 433-2824 x 700
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