Re: Query run in 27s with 15.2 vs 37ms with 14.6

On Tue, Feb 21, 2023 at 9:22 AM David Rowley <dgrowleyml@gmail.com> wrote:

> On Tue, 21 Feb 2023 at 14:38, Charles <peacech@gmail.com> wrote:
> > Disabling mergejoin on 14.6 and disabling nestedloop on 15.2 causes both
> to use hashjoin where it runs for 37ms in 14.6 and 208ms in 15.2.
> >
> > 14.6:
> >                           ->  Parallel Index Scan using
> idx_stock_price_date on stock_price  (cost=0.43..59671.39 rows=81248
> width=13) (actual time=0.021..0.689 rows=1427 loops=3)
> >                                 Index Cond: (date > $0)
> >                                 Filter: (value > 0)
> >                                 Rows Removed by Filter: 222
>
> > 15.2:
> >                                       ->  Parallel Seq Scan on
> stock_price  (cost=0.00..64038.54 rows=91275 width=13) (actual
> time=130.043..173.124 rows=1427 loops=3)
> >                                             Filter: ((value > 0) AND
> (date > $0))
> >                                             Rows Removed by Filter:
> 906975
>
> The difference in the two above fragments likely accounts for the
> majority of the remaining performance difference.  Possibly 15.2 is
> using a Seq Scan because it's estimating slightly more rows from
> stock_price for these two quals. For the date > $0 qual, the stats
> don't really help as the planner does not know what $0 will be during
> planning, so it'll just assume that the selectivity is 1/3rd of rows.
> For the value > 0, there could be some variation there just between
> ANALYZE runs. That might be enough to account for the difference in
> estimate between 14 and 15.
>
> You might also want to check that effective_cache_size is set to
> something realistic on 15. random_page_cost is also a factor for index
> scan vs seq scan.
>

Thank you for the hint. I think this is it. random_page_cost was set to 1.1
in 14.6 (the data directory is located in a ssd). When upgrading to 15.2 I
thought that ssd random seek time is not that close to sequential seek
time, so I revert it back to 4.

The new timings are
random_page_cost = 1.1 with no extended stats = still 27 seconds
random_page_cost = 1.1 with materialized cte = 92ms
random_page_cost = 1.1 with extended stats = 33ms
random_page_cost = 1.1 with extended stats and materialized cte = 33ms