Re: SET TRANSACTION SNAPSHOT sounds like replicated environment but isn't

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From: David G. Johnston <[email protected]>
To: [email protected]
To: Pg Docs <[email protected]>
Subject: Re: SET TRANSACTION SNAPSHOT sounds like replicated environment but isn't
Date: Wed, 11 May 2022 10:00:15 -0700
Message-ID: <CAKFQuwaxozDM-+P-nKPY1aBZ0feckRQbAQJ=d8HJtv8-E8TzQg@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>

On Wed, May 11, 2022 at 7:33 AM PG Doc comments form <[email protected]>
wrote:

> The following documentation comment has been logged on the website:
>
> Page: https://www.postgresql.org/docs/14/sql-set-transaction.html
> Description:
>
> Hello,
>
> the wording on the SET TRANSACTION SNAPSHOT left me a bit confused. It says
> "To begin a new transaction with the same snapshot as an already existing
> transaction" and it feels like basically taking over an existing
> session/transaction or being able to replicate a transaction from that
> snapshot on.
>

I'm not sure what a final documentation patch might look like but this
section can rightfully assume some prior knowledge about how the system
works with regards to snapshots.  In particular, a snapshot is effectively
a single value that a session maintains that lets it compute whether a
given transaction id is visible or not.  So, "whenever you give me data
only do so up to this point-in-time".  Understanding that dynamic should
lead one to conclude that just having a shared since of "point-in-time"
doesn't have anything to do with permissions to interact with specific
objects in the first place.  And, for temporary tables, to break the
session isolation that it is assumed the reader is aware of.

David J.

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