On Fri, Jan 13, 2012 at 11:30 AM, Fujii Masao <masao.fujii@gmail.com> wrote:
> I found that even read transaction waits for sync rep when it generates
> WAL records even if XID is not assigned. For example, imagine the case
> where SELECT query does a heap clean operation and generates
> XLOG_HEAP2_CLEAN record. ISTM that such a read transaction doesn't
> need to wait for sync rep because that's not visible to the client... Can
> we skip waiting for sync rep if XID is not assigned?
Your example of XLOG_HEAP2_CLEAN records is a good one but there are
other record types and circumstances, as described in the comment near
the top of RecordTransactionCommit
/* * If we didn't create XLOG entries, we're done here; otherwise we * should flush those entries the same
asa commit record. (An * example of a possible record that wouldn't cause an XID to be * assigned is a
sequenceadvance record due to nextval() --- we want * to flush that to disk before reporting commit.) */ if
(!wrote_xlog) goto cleanup;
Perhaps there is a case to say that sequences don't need to be flushed
if all they did was do nextval but no change was associated with that,
I'm not sure.
So I think there is a case for optimisation using finer grained decision making.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
