Re: Add REPACK progress phases for logical decoding setup

> I've reviewed the 0001 patch first as it's an open item. There are
> some minor comments:

Thanks!

> - expected to be very brief.
> + expected to be very brief, except for
> + <command>REPACK (CONCURRENTLY)</command>, where it also covers enabling
> + logical decoding and then initializing it. Enabling has nothing to do
> + if <xref linkend="guc-effective-wal-level"/> is already
> + <literal>logical</literal>; otherwise it waits for all backends to
> + acknowledge that they started writing the additional WAL information
> + that logical decoding requires. Initializing then waits for the
>
> The logical decoding activation waits for all processes to acknowledge
> it, not only backends.

Fair. I updated it.

> + transactions that had already been assigned a transaction ID to end, so
> + this phase can last as long as the longest of those transactions.
>
> During the logical decoding initialization, the snapshot builder also
> has to wait for transactions that started after the initialization
> began (i.e., those that started while it was in the BUILDING_SNAPSHOT
> state). So this phase can last longer than the longest of the
> transactions that
> were running when it started. How about rephrasing it as follows?
>
> expected to be very brief, except for
> <command>REPACK (CONCURRENTLY)</command>, where it also covers enabling
> logical decoding and then initializing it. Enabling has nothing to do
> if <xref linkend="guc-effective-wal-level"/> is already
> <literal>logical</literal>; otherwise it waits for all processes to
> acknowledge that they started writing the additional WAL information
> that logical decoding requires. Initializing then waits for the
> transactions that have been assigned a transaction ID to end,
> including ones that started while this wait was in progress, so this
> phase can last longer than the longest transaction running when the
> command started.

Good point. I used what you provided and I realized that in my v1 I was also
specifically calling out that "waits for the transactions that have
been assigned a transaction ID to end"
also includes other REPACK CONCURRENTLY operations. This may not be obvious,
so it's worth a callout.

--
Sami Imseih
Amazon Web Services (AWS)