On Wed, Sep 23, 2015 at 3:21 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> "Shulgin, Oleksandr" <oleksandr.shul...@zalando.de> writes: > > On Tue, Sep 22, 2015 at 11:56 PM, Alvaro Herrera < > alvhe...@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. -- Alex
