Re: BUG #18588: Cannot force/let database use parallel execution in simple case.

On Thu, Aug 22, 2024 at 10:01 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Maxim Boguk <maxim(dot)boguk(at)gmail(dot)com> writes:
> > Ok there are reproducer test case on the stock postgresql 16.4 config:
>
> Hmm, you can see both behaviors on the small version of t1, just by
> varying the comparison constant in the WHERE clause. For me, it'll
> use only one worker with "where a<1", and not parallelize at all
> with "where a<0". It looks like it's deciding that it's not worth
> starting workers when too few rows are expected to be returned. That
> would be unsurprising with a normal setting of parallel_setup_cost,
> but it does seem odd with parallel_setup_cost=0.
>
> In any case, I think this isn't about the big table being big but
> about changing the range of values of "a", which changes the
> selectivity of "where a<10" 100-fold.
>
> (I tested on HEAD not v16)
>
> regards, tom lane
>

I especially designed both tests in a way that the expected amount of rows
- is the same in both cases.

With sufficiently large table - I didn't manage find a combinations of
settings to force parallel execution (im my real case - the database huge
and mostly cold on ssd raid - so parallel execution provides almost linear
speedup with amount of parallel workers even on 1000 rows... not talking
about 10k-10m rows, but I cannot convince the PostgreSQL use parallel index
scan for 2m returned rows case).

--
Maxim Boguk
Senior Postgresql DBA

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