On 11/04/2014 07:31 AM, Álvaro Hernández Tortosa wrote:
> Thank you for your comment, Tom. However I think this behavior, as
> seen from a user perspective, it's not the expected one.
That may be the case, but I think it's the SQL-standard behaviour, so we
can't really mess with it.
The spec requires SET TRANSACTION ISOLATION, and you can't implement
that if you take a snapshot at BEGIN.
> If it is still the intended behavior, I think it should be clearly
> documented as such, and a recommendation similar to "issue a 'SELECT 1'
> right after BEGIN to freeze the data before any own query" or similar
> comment should be added. Again, as I said in my email, the documentation
> clearly says that "only sees data committed before the transaction
> began". And this is clearly not the real behavior.
It's more of a difference in when the transaction "begins".
Arguably, "BEGIN" says "I intend to begin a new transaction with the
next query" rather than "immediately begin executing a new transaction".
This concept could be clearer in the docs.
> Sure, there are, that was the link I pointed out, but I found no
> explicit mention to the fact that I'm raising here.
I'm sure it's documented *somewhere*, in that I remember reading about
this detail in the docs, but I can't find _where_ in the docs.
It doesn't seem to be in:
http://www.postgresql.org/docs/current/static/transaction-iso.html
where I'd expect.
In any case, we simply cannot take the snapshot at BEGIN time, because
it's permitted to:
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
in a DB that has default serializable isolation or has a SET SESSION
CHARACTERISTICS isolation mode of serializable. Note that SET
TRANSACTION is SQL-standard.
AFAIK deferring the snapshot that's consistent with other RDBMSes that
use snapshots, too.
The docs of that command allude to, but doesn't explicitly state, the
behaviour you mention.
http://www.postgresql.org/docs/current/static/sql-set-transaction.html
-- Craig Ringer http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
