"Shulgin, Oleksandr" <oleksandr.shulgin@zalando.de> writes:
> On Tue, Sep 22, 2015 at 11:56 PM, Alvaro Herrera <alvherre@2ndquadrant.com>
> wrote:
>> It looks like a bug to me, but I think it might destabilize approved
>> execution plans(*), so it may not be such a great idea to back patch
>> branches that are already released. I think HEAD + 9.5 is good.
>>
>> (*) I hear there are even applications where queries and their approved
>> execution plans are kept in a manifest, and plans that deviate from that
>> raise all kinds of alarms. I have never seen such a thing ...
> Ugh. Anyway, do you expect any plans to change only due to avg. width
> estimation being different? Why would that be so?
Certainly, eg it could affect a decision about whether to use a hash join
or hash aggregation through changing the planner's estimate of the
required hashtable size. We wouldn't be bothering to track that data if
it didn't affect plans.
Personally I think Alvaro's position is unduly conservative: to the extent
that plans change it'd likely be for the better. But I'm not excited
enough to fight hard about it.
regards, tom lane
