Re: Query plan prefers hash join when nested loop is much faster

On Tue, Aug 25, 2020 at 12:36 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Tue, 25 Aug 2020 at 22:10, iulian dragos
> <iulian(dot)dragos(at)databricks(dot)com> wrote:
> > Thanks for the tip! Indeed, `n_distinct` isn't right. I found it in
> pg_stats set at 131736.0, but the actual number is much higher: 210104361.
> I tried to set it manually, but the plan is still the same (both the actual
> number and a percentage, -0.4, as you suggested):
>
> You'll need to run ANALYZE on the table after doing the ALTER TABLE to
> change the n_distinct. The ANALYZE writes the value to pg_statistic.
> ALTER TABLE only takes it as far as pg_attribute's attoptions.
> ANALYZE reads that column to see if the n_distinct estimate should be
> overwritten before writing out pg_statistic
>

Ah, rookie mistake. Thanks for clarifying this. Indeed, after I ran ANALYZE
the faster plan was selected! Yay!

> Just remember if you're hardcoding a positive value that it'll stay
> fixed until you change it. If the table is likely to grow, then you
> might want to reconsider using a positive value and consider using a
> negative value as mentioned in the doc link.
>

Good point, I went for -0.4 and that seems to be doing the trick!

Thanks a lot for helping out!

>
> David
>