Robert Haas wrote:
> On Sat, Aug 20, 2016 at 4:58 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Jeff Janes <jeff.janes@gmail.com> writes:
> >> On Thu, Aug 18, 2016 at 2:25 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >>> It does know it, what it doesn't know is how many duplicates there are.
> >
> >> Does it know whether the count comes from a parsed query-string list/array,
> >> rather than being an estimate from something else? If it came from a join,
> >> I can see why it would be dangerous to assume they are mostly distinct.
> >> But if someone throws 6000 things into a query string and only 200 distinct
> >> values among them, they have no one to blame but themselves when it makes
> >> bad choices off of that.
> >
> > I am not exactly sold on this assumption that applications have
> > de-duplicated the contents of a VALUES or IN list. They haven't been
> > asked to do that in the past, so why do you think they are doing it?
>
> It's hard to know, but my intuition is that most people would
> deduplicate. I mean, nobody is going to want to their query generator
> to send X IN (1, 1, <repeat a zillion more times>) to the server if it
> could have just sent X IN (1).
Also, if we patch it this way and somebody has a slow query because of a
lot of duplicate values, it's easy to solve the problem by
de-duplicating. But with the current code, people that have the
opposite problem has no way to work around it.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
