Re: Regression in query optimizer when upgrading from 9.4.5 to 9.5.2?

"Erdmann, Markus @ Bellevue" <Markus(dot)Erdmann(at)cbre(dot)com> writes:
> Were trying to debug a performance issue affecting our staging database, and weve narrowed it down to a difference in the query optimizer in 9.5.2. Upgrading to 9.5 is important for us because we need the ability to import foreign schemas.

I think the core of the problem is the large rowcount estimation error
here:

> -> Bitmap Index Scan on transactions_transaction_c69e55a4 (cost=0.00..18.02 rows=161 width=0) (actual time=20.153..20.153 rows=269021 loops=1)
> Index Cond: ((date_created >= '2010-01-01'::date) AND (date_created <= '2015-12-31'::date))

That's a pretty simple condition and it's hard to believe that 9.5 does it
much differently than 9.4 did. Perhaps you forgot to ANALYZE, or were
using a larger statistics target in the 9.4 installation?

regards, tom lane