On Fri, Jan 13, 2012 at 11:30 AM, Fujii Masao <masao.fu...@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 as a commit record. (An * example of a possible record that wouldn't cause an XID to be * assigned is a sequence advance 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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers