| From: | Alexey Bashtanov <bashtanov(at)imap(dot)cc> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | pgsql-bugs(at)postgresql(dot)org |
| Subject: | Re: planner weirdness: a join uses nestloop with checking condition when there are two subplan-or-hashed subqueries |
| Date: | 2020-05-22 21:08:22 |
| Message-ID: | a25b34f8-f3fe-3a74-044a-3ad5ad6b26ce@imap.cc |
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| Lists: | pgsql-bugs |
Hi Tom,
On 25/02/2020 20:34, Tom Lane wrote:
> cost_qual_eval_walker is not very bright about what to do with
> the AlternativeSubPlan constructs. It looks like it's assuming
> that the non-hashed alternatives will be chosen, which they aren't
> (if they were, this estimate might not be so far out of line).
> But we can't just switch it to make the other assumption, because
> that would skew the results for other cases.
What do you think of making it take both into account?
That's an approximation of a piecewise linear function by a linear one,
so it cannot be very precise, but I think it's better than the current one.
Best, Alex
| Attachment | Content-Type | Size |
|---|---|---|
| alternatives-cost-as-geom-mean-v1.patch | text/x-patch | 3.0 KB |
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