Re: Asynchronous Append on postgres_fdw nodes.

On Mon, Apr 26, 2021 at 3:01 PM Andrey V. Lepikhov
<a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
> While studying the capabilities of AsyncAppend, I noticed an
> inconsistency with the cost model of the optimizer:

> Here I see two problems:
> 1. Cost of an AsyncAppend is the same as cost of an Append. But
> execution time of the AsyncAppend for three remote partitions has more
> than halved.
> 2. Cost of an AsyncAppend looks as a sum of the child ForeignScan costs.

Yeah, we don’t adjust the cost for async Append; it’s the same as that
for sync Append. But I don’t see any issue as-is, either. (It’s not
that easy to adjust the cost to an appropriate value in the case of
postgres_fdw, because in that case the cost would vary depending on
which connections are used for scanning foreign tables [1].)

> I haven't ideas why it may be a problem right now. But I can imagine
> that it may be a problem in future if we have alternative paths: complex
> pushdown in synchronous mode (a few rows to return) or simple
> asynchronous append with a large set of rows to return.

Yeah, I think it’s better if we could consider async append paths and
estimate the costs for them accurately at path-creation time, not
plan-creation time, because that would make it possible to use async
execution in more cases, as you pointed out. But I left that for
future work, because I wanted to make the first cut simple.

Thanks for the review!

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CAPmGK15i-OyCesd369P8zyBErjN_T18zVYu27714bf_L%3DCOXew%40mail.gmail.com