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From: Antonin Houska <ah@cybertec.at>
To: pgsql-hackers@postgresql.org
Subject: Re: snapbuild woes
Date: Fri, 09 Jun 2017 09:25:34 +0200
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Andres Freund <andres@anarazel.de> wrote:

> Looking at 0001:
> - GetOldestSafeDecodingTransactionId() only guarantees to return an xid
>   safe for decoding (note how procArray->replication_slot_catalog_xmin
>   is checked), not one for the initial snapshot - so afaics this whole
>   exercise doesn't guarantee much so far.

I happen to use CreateInitDecodingContext() in an extension, so I had to think
what the new argumen exactly means (as for the incompatibility between PG
9.6.2 and 9.6.3, I suppose preprocessor directives can handle it).

One thing I'm failing to understand is: if TRUE is passed for
need_full_snapshot, then slot->effective_xmin receives the result of

    GetOldestSafeDecodingTransactionId(need_full_snapshot)

but this does include "catalog xmin".

If the value returned is determined by an existing slot which has valid
effective_catalog_xmin and invalid effective_xmin (i.e. that slot only
protects catalog tables from VACUUM but not the regular ones), then the new
slot will think it (i.e. the new slot) protects even non-catalog tables, but
that's no true.

Shouldn't the xmin_horizon be computed by this call instead?

    GetOldestSafeDecodingTransactionId(!need_full_snapshot);

(If so, I think "considerCatalog" argument name would be clearer than
"catalogOnly".)

-- 
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at


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