Date: Mon, 3 Aug 2015 01:35:50 +0000 (UTC)
From: Kevin Grittner <kgrittn@ymail.com>
Reply-To: Kevin Grittner <kgrittn@ymail.com>
To: Dmitry Igrishin <dmitigr@gmail.com>,
 pgsql-docs <pgsql-docs@postgresql.org>
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Subject: Re: Confused by example in 13.2.2
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Dmitry Igrishin <dmitigr@gmail.com> wrote:

> I'm confused a bit by example in 13.2.2:

In current docs that is the "Repeatable Read Isolation Level"
section.

> "For example, even a read only transaction at this level may see a
> control record updated to show that a batch has been completed but
> not see one of the detail records which is logically part of the batch
> because it read an earlier revision of the control record."
>
> Could anyone clarify this example? And what is "control record"?

Yeah, that sentence doesn't really have enough context. There was
discussion around this example in the Wiki:

https://wiki.postgresql.org/wiki/SSI#Deposit_Report

... and with that fresh in everyone's memory it seemed to make
sense at the time. Would this change make it clear to you?:

"For example, you might have a workload involving batches, with a
control table having a record to describe the state of each batch
and another table with the detail; a read only transaction at this
isolation level could see a control record updated to show that a
batch has been completed but not see one of the detail records
which is logically part of the batch because the transaction
inserting the detail read an earlier version of the control
record."

Hopefully the example from the Wiki makes it clear. There has been
some discussion about whether to link to the Wiki or bring an
example like that into the documentation; but we usually don't
link to Wiki pages and the example is somewhat long to include
in-line in the documentation. Any thoughts on the best solution to
that?

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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