Date: Fri, 2 Jul 2021 15:56:30 -0400
From: Bruce Momjian <bruce@momjian.us>
To: mossa708496@gmail.com, pgsql-docs@lists.postgresql.org
Subject: Re: docs issue in transactions section
Message-ID: <20210702195630.GC25522@momjian.us>
References: <162525307568.700.736681752884243253@wrigleys.postgresql.org>
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On Fri, Jul  2, 2021 at 07:11:15PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
> 
> Page: https://www.postgresql.org/docs/13/transaction-iso.html
> Description:
> 
> 13.2.1. Read Committed Isolation Level
> Read Committed is the default isolation level in PostgreSQL. When a
> transaction uses this isolation level, a SELECT query (without a FOR
> UPDATE/SHARE clause) sees only data committed before the query began; it
> never sees either uncommitted data or changes committed during query
> execution by concurrent transactions. In effect, a SELECT query sees a
> snapshot of the database as of the instant the query begins to run. However,
> SELECT does see the effects of previous updates executed within its own
> transaction, even though they are not yet committed. Also note that two
> successive SELECT commands can see different data, even though they are
> within a single transaction, if other transactions commit changes after the
> first SELECT starts and before the second SELECT starts.
> 
> I don't understand this very clearly: " it never sees either uncommitted
> data or changes committed during query execution by concurrent
> transactions." and this statement " Also note that two successive SELECT
> commands can see different data, even though they are within a single
> transaction, if other transactions commit changes after the first SELECT
> starts and before the second SELECT starts."

Uh, what is your question?  Does page 11 of this help?

	https://momjian.us/main/writings/pgsql/mvcc.pdf#page=11

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.