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From: Andres Freund <[email protected]>
To: Drouvot, Bertrand <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Thomas Munro <[email protected]>
Cc: Alvaro Herrera <[email protected]>
Cc: Ibrar Ahmed <[email protected]>
Cc: Amit Khandekar <[email protected]>
Cc: [email protected]
Cc: tushar <[email protected]>
Cc: Rahila Syed <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Minimal logical decoding on standbys
Date: Mon, 23 Jan 2023 16:46:12 -0800
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
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Hi,

On 2023-01-19 10:43:27 +0100, Drouvot, Bertrand wrote:
> > > With a reload in place in my testing, now I notice that the catalog_xmin
> > > is updated on the primary physical slot after logical slots invalidation
> > > when reloading hot_standby_feedback from "off" to "on".
> > > 
> > > This is not the case after a re-start (aka catalog_xmin is NULL).
> > > 
> > > I think a re-start and reload should produce identical behavior on
> > > the primary physical slot. If so, I'm tempted to think that the catalog_xmin
> > > should be updated in case of a re-start too (even if all the logical slots are invalidated)
> > > because the slots are not dropped yet. What do you think?
> > 
> > I can't quite follow the steps leading up to the difference. Could you list
> > them in a bit more detail?
> > 
> > 
> 
> Sure, so with:
> 
> 1) hot_standby_feedback set to off on the standby
> 2) create 2 logical replication slots on the standby and activate one
> 3) Invalidate the logical slots on the standby with VACUUM FULL on the primary
> 4) change hot_standby_feedback to on on the standby
> 
> If:
> 
> 5) pg_reload_conf() on the standby, then on the primary we get a catalog_xmin
> for the physical slot that the standby is attached to:
> 
> postgres=# select slot_type,xmin,catalog_xmin  from pg_replication_slots ;
>  slot_type | xmin | catalog_xmin
> -----------+------+--------------
>  physical  |  822 |          748
> (1 row)

How long did you wait for this to change? I don't think there's anything right
now that'd force a new hot-standby-feedback message to be sent to the primary,
after slots got invalidated.

I suspect that if you terminated the walsender connection on the primary,
you'd not see it anymore either?

If that isn't it, something is broken in InvalidateObsolete...


> No, but a question still remains to me:
> 
> Given the fact that the row removal case is already done
> in the next test (aka Scenario 2), If we want to replace the "vacuum full" test
> on the database (done in Scenario 1) with a cheaper one at the table level,
> what could it be to guarantee an invalidation?
> 
> Same as scenario 2 but with "vacuum full pg_class" would not really add value
> to the tests, right?

A database wide VACUUM FULL is also just a row removal test, no? I think it
makes sense to test that both VACUUM and VACUUM FULL both trigger conflicts,
because they internally use *very* different mechanisms.  It'd probably be
good to test at least conflicts triggered due to row removal via on-access
pruning as well. And perhaps also for btree killtuples.  I think those are the
common cases for catalog tables.

Greetings,

Andres Freund

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