Re: Serialization questions

On Tue, Aug 20, 2019 at 4:47 PM Alex <zhihui(dot)fan1213(at)gmail(dot)com> wrote:

> Before understanding how postgres implements the serializable isolation
> level (I have see many paper related to it), I have question about how it
> should be.
>
>
> I mainly read the ideas from
> https://www.postgresql.org/docs/11/transaction-iso.html.
>
>
> In fact, this isolation level works exactly the same as Repeatable Read
> except that it monitors for conditions which could make execution of a
> concurrent set of serializable transactions behave in a manner inconsistent
> with all possible serial (one at a time) executions of those transactions.
>
>
>
> in repeatable read, every statement will use the transaction start
> timestamp, so is it in serializable isolation level?
>
>
> When relying on Serializable transactions to prevent anomalies, it is
> important that any data read from a permanent user table not be considered
> valid until the transaction which read it has successfully committed. This
> is true even for read-only transactions ...
>
>
> What does the "not be considered valid" mean? and if it is a read-only
> transaction (assume T1), I think it is ok to let other transaction do
> anything with the read set of T1, since it is invisible to T1(use the
> transaction start time as statement timestamp).
>

first issue "set default_transaction_isolation to 'serializable';" on the
both sessions, then run:

Session 1: begin; select * from t; (2 rows selected);
Session 2: delete from t; (committed automatically)
Session 1: commit; (commit successfully).

looks the reads in session 1 has no impact on the session 2 at all which is
conflicted with the document

> Thanks
>