On 9/24/24 05:59, Ron Johnson wrote:
On Tue, Sep 24, 2024 at 8:29 AM David G. Johnston
<david.g.johnston@xxxxxxxxx <mailto:david.g.johnston@xxxxxxxxx>> wrote:
On Monday, September 23, 2024, Wizard Brony <wizardbrony@xxxxxxxxx
<mailto:wizardbrony@xxxxxxxxx>> 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
--
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Don't boil me, I'm still alive.
<Redacted> crustacean!
--
Adrian Klaver
adrian.klaver@xxxxxxxxxxx
