Re: Repeatable Read Isolation Level "transaction start time" - Mailing list pgsql-general

On 9/24/24 05:59, Ron Johnson wrote:
> On Tue, Sep 24, 2024 at 8:29 AM David G. Johnston
> <david.g.johnston@gmail.com <mailto:david.g.johnston@gmail.com>> wrote:
>
>     On Monday, September 23, 2024, Wizard Brony <wizardbrony@gmail.com
>     <mailto:wizardbrony@gmail.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