Обсуждение: Why do spgbuildempty(), btbuildempty(), and blbuildempty() use smgrwrite()?

If you enable the CHECK_WRITE_VS_EXTEND-protected assert in mdwrite(),
you'll trip it anytime you exercise btbuildempty(), blbuildempty(), or
spgbuildempty().

In this case, it may not make any meaningful difference if smgrwrite()
or smgrextend() is called (_mdfd_getseg() behavior won't differ even
with the different flags, so really only the FileWrite() wait event will
be different).
However, it seems like it should still be changed to call smgrextend().
Or, since they only write a few pages, why not use shared buffers like
gistbuildempty() and brinbuildempty() do?

I've attached a patch to move these three into shared buffers.

And, speaking of spgbuildempty(), there is no test exercising it in
check-world, so I've attached a patch to do so. I wasn't sure if it
belonged in spgist.sql or create_index_spgist.sql (on name alone, seems
like the latter but based on content, seems like the former).

- Melanie

At Wed, 2 Mar 2022 20:07:14 -0500, Melanie Plageman <melanieplageman@gmail.com> wrote in 
> If you enable the CHECK_WRITE_VS_EXTEND-protected assert in mdwrite(),
> you'll trip it anytime you exercise btbuildempty(), blbuildempty(), or
> spgbuildempty().
> 
> In this case, it may not make any meaningful difference if smgrwrite()
> or smgrextend() is called (_mdfd_getseg() behavior won't differ even
> with the different flags, so really only the FileWrite() wait event will
> be different).
> However, it seems like it should still be changed to call smgrextend().
> Or, since they only write a few pages, why not use shared buffers like
> gistbuildempty() and brinbuildempty() do?
> 
> I've attached a patch to move these three into shared buffers.
> 
> And, speaking of spgbuildempty(), there is no test exercising it in
> check-world, so I've attached a patch to do so. I wasn't sure if it
> belonged in spgist.sql or create_index_spgist.sql (on name alone, seems
> like the latter but based on content, seems like the former).
> 
> - Melanie

I didn't dig into your specific isssue, but I'm mildly opposed to
moving them onto shared buffers.  They are cold images of init-fork,
which is actually no-use for active servers.  Rather I'd like to move
brinbuildempty out of shared buffers considering one of my proposing
patches..

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center