Re: Repeatable Read Isolation Level "transaction start time"

On Tue, Sep 24, 2024 at 12:06 PM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> 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
>

Makes sense. Never would have occurred to me to try and change the
isolation level using a second SET TRANSACTION statement, though.

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