Re: Slow recursive CTE query questions, with row estimate and n_distinct issues

Michael Lewis <mlewis@entrata.com> writes:

> On Mon, Dec 28, 2020 at 7:51 AM Christopher Baines <mail@cbaines.net> wrote:
>
>> derivation_inputs:
>>   COUNT(*):  285422539
>>   reltuples: 285422528
>>
>>   derivation_id:
>>     COUNT(DISTINCT): 7508610
>>     n_distinct:      4336644 (~57% of the true value)
>>
>>   derivation_output_id:
>>     COUNT(DISTINCT): 5539406
>>     n_distinct:      473762 (~8% of the true value)
>>
>
> If you expect the ratio of distinct of derivation_output_id values to be
> roughly linear going forward, you can set a custom value for n_distinct on
> the column (currently seems like -.0194, aka distinct count
> of derivation_output_id divided by reltuples of the table). You could also
> do this analysis every month or six and set the custom value as needed.
>
> https://www.postgresql.org/docs/current/sql-altertable.html
>
> I am not sure if it will resolve your query problems though.

Thanks Michael, I didn't realise a custom value could be set, but I'll
look in to this.

I actually managed to speed the query up enough by increasing
work_mem/shared_buffers. I didn't realise one of the sequential scans
was executing 14 times, but giving PostgreSQL more resources means it
just executes once, which really helps.

Thanks again,

Chris