On Tue, Aug 25, 2020 at 12:36 PM David Rowley <dgrowleyml@xxxxxxxxx> wrote:
On Tue, 25 Aug 2020 at 22:10, iulian dragos
<iulian.dragos@xxxxxxxxxxxxxx> 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
