On Wed, Sep 23, 2015 at 3:21 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
"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.
Yeah, I can see now, as I was studying the hash node code today intensively for an unrelated reason.
I also believe that given that we are going to have more accurate stats, the plan changes in this case hopefully are a good thing.
