Alvaro Herrera <[EMAIL PROTECTED]> writes:
> Tom Lane wrote:
>> I'm wondering about doing something similar to what
>> TransactionIdIsInProgress does, ie, make use of the PGPROC lists
>> of live subtransactions.

> Well, that sounds awfully more expensive than setting
> local-to-my-database Xmins as well as global (all databases) Xmins :-)

Only when you've got a lot of subtransactions.  The point of this
discussion is to optimize for the few-or-none case.  In any case,
the problem with the local/global bit was that you'd be expending
foreground-process cycles without any foreground-process return.
Being able to use a snapshot without consulting pg_subtrans will
certainly buy back some cycles...

                        regards, tom lane

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