Обсуждение: Isolation levels READ UNCOMMITTED and REPEATABLE READ

I think we could allow users to set the transaction isolation level to
READ UNCOMMITTED or REPEATABLE READ and internally behave like READ
COMMITTED or SERIALIZABLE, respectively.  The SQL standard seems to allow
this:

[speaking about SET TRANSACTION]
        5) The isolation level of TXN is set to an implementation-defined           isolation level that will not
exhibitany of the phenomena that           the explicit or implicit <level of isolation> would not exhibit,
asspecified in Table 10, "SQL-transaction isolation levels and           the three phenomena".
 

This says that the SQL implementation can always isolate more than the
user requested, just not less.

If we did this, it would make the SQL interface more complete at little
cost.

Comments?

-- 
Peter Eisentraut   peter_e@gmx.net

On Thu, 9 Oct 2003, Peter Eisentraut wrote:

> I think we could allow users to set the transaction isolation level to
> READ UNCOMMITTED or REPEATABLE READ and internally behave like READ
> COMMITTED or SERIALIZABLE, respectively.  The SQL standard seems to allow
> this:

Why not.

I would like a warning to be outputted also, but other then that, why not.

-- 
/Dennis

Peter Eisentraut <peter_e@gmx.net> writes:
> I think we could allow users to set the transaction isolation level to
> READ UNCOMMITTED or REPEATABLE READ and internally behave like READ
> COMMITTED or SERIALIZABLE, respectively.  The SQL standard seems to allow
> this:

My reading is that the spec *requires* this.  We have not done it
because there was some feeling that people would be confused if the
isolation levels appeared to exist but didn't really work as they
expected.  But a notice or something could help address that concern.
        regards, tom lane

Added to TODO:
* Allow more ISOLATION LEVELS to be accepted, but issue a  warning for them


---------------------------------------------------------------------------

Tom Lane wrote:
> Peter Eisentraut <peter_e@gmx.net> writes:
> > I think we could allow users to set the transaction isolation level to
> > READ UNCOMMITTED or REPEATABLE READ and internally behave like READ
> > COMMITTED or SERIALIZABLE, respectively.  The SQL standard seems to allow
> > this:
> 
> My reading is that the spec *requires* this.  We have not done it
> because there was some feeling that people would be confused if the
> isolation levels appeared to exist but didn't really work as they
> expected.  But a notice or something could help address that concern.
> 
>             regards, tom lane
> 
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