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Re: Using each rel as both outer and inner for JOIN_ANTI - Mailing list pgsql-hackers

From Richard Guo
Subject Re: Using each rel as both outer and inner for JOIN_ANTI
Date
Msg-id CAMbWs48TAs+65GsK989jXKG6cmcD3yRwa0P910sXobzmuQ9UdQ@mail.gmail.com
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In response to Re: Using each rel as both outer and inner for JOIN_ANTI  (Tom Lane <tgl@sss.pgh.pa.us>)
Responses Re: Using each rel as both outer and inner for JOIN_ANTI  (Thomas Munro <thomas.munro@gmail.com>)
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On Thu, Apr 6, 2023 at 1:06 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
This:

> +#if 0
>         /* If any limit was set to zero, the user doesn't want a
> parallel scan. */
>         if (parallel_workers <= 0)
>                 return;
> +#endif

seems like it adds a lot of new paths with a lot lower chance
of win, but maybe we could tighten the conditions to improve
the odds?

Seems it wins if the parallel scan becomes part of a hash join in final
plan.  I wonder if we have a way to know that in this early stage.

BTW, zero parallel_workers seems would break some later assumptions, so
we may need to give it a meaningful number if we want to do in this way.

Thanks
Richard

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