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From: Matthias van de Meent <boekewurm+postgres@gmail.com>
Date: Wed, 7 Aug 2024 01:40:51 +0200
Message-ID: <CAEze2Wj9SgwOpe_1CWnS_D-txQaQyXArR=dm4DTnha93=yua4g@mail.gmail.com>
Subject: Re: Revisiting {CREATE INDEX, REINDEX} CONCURRENTLY improvements
To: Michail Nikolaev <michail.nikolaev@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Andrey Borodin <amborodin86@gmail.com>, 
	Melanie Plageman <melanieplageman@gmail.com>
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On Tue, 11 Jun 2024 at 10:58, Michail Nikolaev
<michail.nikolaev@gmail.com> wrote:
>
> Hello.
>
> I did the POC (1) of the method described in the previous email, and it looks promising.
>
> It doesn't block the VACUUM, indexes are built about 30% faster (22 mins vs 15 mins).

That's a nice improvement.

> Additional index is lightweight and does not produce any WAL.

That doesn't seem to be what I see in the current patchset:
https://github.com/postgres/postgres/compare/master...michail-nikolaev:postgres:new_index_concurrently_approach#diff-cc3cb8968cf833c4b8498ad2c561c786099c910515c4bf397ba853ae60aa2bf7R311

> I'll continue the more stress testing for a while. Also, I need to restructure the commits (my path was no direct) into some meaningful and reviewable patches.

While waiting for this, here are some initial comments on the github diffs:

- I notice you've added a new argument to
heapam_index_build_range_scan. I think this could just as well be
implemented by reading the indexInfo->ii_Concurrent field, as the
values should be equivalent, right?

- In heapam_index_build_range_scan, it seems like you're popping the
snapshot and registering a new one while holding a tuple from
heap_getnext(), thus while holding a page lock. I'm not so sure that's
OK, expecially when catalogs are also involved (specifically for
expression indexes, where functions could potentially be updated or
dropped if we re-create the visibility snapshot)

- In heapam_index_build_range_scan, you pop the snapshot before the
returned heaptuple is processed and passed to the index-provided
callback. I think that's incorrect, as it'll change the visibility of
the returned tuple before it's passed to the index's callback. I think
the snapshot manipulation is best added at the end of the loop, if we
add it at all in that function.

- The snapshot reset interval is quite high, at 500ms. Why did you
configure it that low, and didn't you make this configurable?

- You seem to be using WAL in the STIR index, while it doesn't seem
that relevant for the use case of auxiliary indexes that won't return
any data and are only used on the primary. It would imply that the
data is being sent to replicas and more data being written than
strictly necessary, which to me seems wasteful.

- The locking in stirinsert can probably be improved significantly if
we use things like atomic operations on STIR pages. We'd need an
exclusive lock only for page initialization, while share locks are
enough if the page's data is modified without WAL. That should improve
concurrent insert performance significantly, as it would further
reduce the length of the exclusively locked hot path.

Kind regards,

Matthias van de Meent
Neon (https://neon.tech)