Re: More stable query plans via more predictable column statistics

Hi,

On Mon, 2016-03-07 at 12:17 +0100, Shulgin, Oleksandr wrote:
> On Fri, Mar 4, 2016 at 7:27 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> On Thu, Mar 3, 2016 at 2:48 AM, Shulgin, Oleksandr
> <oleksandr(dot)shulgin(at)zalando(dot)de> wrote:
> > On Wed, Mar 2, 2016 at 7:33 PM, Alvaro Herrera
> <alvherre(at)2ndquadrant(dot)com>
> > wrote:
> >> Shulgin, Oleksandr wrote:
> >>
> >> > Alright. I'm attaching the latest version of this patch
> split in two
> >> > parts: the first one is NULLs-related bugfix and the
> second is the
> >> > "improvement" part, which applies on top of the first
> one.
> >>
> >> So is this null-related bugfix supposed to be backpatched?
> (I assume
> >> it's not because it's very likely to change existing
> plans).
> >
> > For the good, because cardinality estimations will be more
> accurate in these
> > cases, so yes I would expect it to be back-patchable.
>
> -1. I think the cost of changing existing query plans in back
> branches is too high. The people who get a better plan never
> thank
> us, but the people who (by bad luck) get a worse plan always
> complain.
>
>
> They might get that different plan when they upgrade to the latest
> major version anyway. Is it set somewhere that minor version upgrades
> should never affect the planner? I doubt so.

Major versions are supposed to add features, which may easily result in
plan changes. Moreover people are expected to do more thorough testing
on major version upgrade, so they're more likely to spot them.

OTOH minor versions are bugfix-only relases, and sometimes the fixes are
security related and people are supposed to install them ASAP. So many
people simply upgrade them without much additional testing and while we
can't promise any of the fixes won't change the plans, we kinda try to
minimize such cases.

That being said, I don't have a clear opinion whether to backpatch this.
I think that it's clearly a bug (especially the first part dealing with
NULL values), and it'd be good to backpatch that. OTOH I can't really
quantify the risks of changing some plans to worse ones.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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