From: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
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To: | Andres Freund <andres(at)anarazel(dot)de> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Michael Paquier <michael(at)paquier(dot)xyz>, Dmitry Dolgov <9erthalion6(at)gmail(dot)com>, James Coleman <jtc331(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: remove spurious CREATE INDEX CONCURRENTLY wait |
Date: | 2020-11-24 21:38:09 |
Message-ID: | 20201124213809.GA14227@alvherre.pgsql |
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Lists: | pgsql-hackers |
On 2020-Nov-23, Andres Freund wrote:
> On 2020-11-23 12:30:05 -0300, Alvaro Herrera wrote:
> > PROC_IN_LOGICAL_DECODING:
> > Oddly enough, I think the reset of PROC_IN_LOGICAL_DECODING in
> > ReplicationSlotRelease might be the most problematic one of the lot.
> > That's because a proc's xmin that had been ignored all along by
> > ComputeXidHorizons, will now be included in the computation. Adding
> > asserts that proc->xmin and proc->xid are InvalidXid by the time we
> > reset the flag, I got hits in pg_basebackup, test_decoding and
> > subscription tests. I think it's OK for ComputeXidHorizons (since it
> > just means that a vacuum that reads a later will remove less rows.) But
> > in GetSnapshotData it is just not correct to have the Xmin go backwards.
>
> I don't think there's a problem. PROC_IN_LOGICAL_DECODING can only be
> set when outside a transaction block, i.e. walsender. In which case
> there shouldn't be an xid/xmin, I think? Or did you gate your assert on
> PROC_IN_LOGICAL_DECODING being set?
Ah, you're right about this one -- I missed the significance of setting
the flag only "when outside of a transaction block" at the time we call
StartupDecodingContext.
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