Re: snapbuild woes

From: Antonin Houska <ah(at)cybertec(dot)at>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: snapbuild woes
Date: 2017-06-09 07:25:34
Message-ID: 32704.1496993134@localhost
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Andres Freund <andres(at)anarazel(dot)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


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?


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

Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt

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