Re: Repeatable Read Isolation Level "transaction start time"

On 9/24/24 05:59, Ron Johnson wrote:
> On Tue, Sep 24, 2024 at 8:29 AM David G. Johnston
> <david(dot)g(dot)johnston(at)gmail(dot)com <mailto:david(dot)g(dot)johnston(at)gmail(dot)com>> wrote:
>
> On Monday, September 23, 2024, Wizard Brony <wizardbrony(at)gmail(dot)com
> <mailto:wizardbrony(at)gmail(dot)com>> wrote:
>
> https://www.postgresql.org/docs/16/transaction-iso.html#XACT-REPEATABLE-READ <https://www.postgresql.org/docs/16/transaction-iso.html#XACT-REPEATABLE-READ>
>
> The PostgreSQL documentation for the Repeatable Read Isolation
> Level states the following:
>
> “UPDATE, DELETE, MERGE, SELECT FOR UPDATE, and SELECT FOR SHARE
> commands behave the same as SELECT in terms of searching for
> target rows: they will only find target rows that were committed
> as of the transaction start time.”
>
> What is defined as the "transaction start time?" When I first
> read the statement, I interpreted it as the start of the
> transaction:
>
> BEGIN;
> SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
>
> But in my testing, I find that according to that statement, the
> transaction start time is actually "the start of the first
> non-transaction-control statement in the transaction" (as
> mentioned earlier in the section). Is my conclusion correct, or
> am I misunderstanding the documentation?
>
>
> Probably, since indeed the transaction cannot start at begin because
> once it does start it cannot be modified.
>
> Huh?

BEGIN;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

I read it as the transaction does not start at BEGIN because if it did
you could not SET TRANSACTION to change it's characteristics.

The docs go into more detail:

https://www.postgresql.org/docs/current/sql-set-transaction.html

The transaction isolation level cannot be changed after the first query
or data-modification statement (SELECT, INSERT, DELETE, UPDATE, MERGE,
FETCH, or COPY) of a transaction has been executed.

So:

begin ;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SET
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET
select * from csv_test ;
[...]
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
ERROR: SET TRANSACTION ISOLATION LEVEL must be called before any query

>
> --
> Death to <Redacted>, and butter sauce.
> Don't boil me, I'm still alive.
> <Redacted> crustacean!

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com