I don't mean to dismiss the potential for further optimization inside XidInMVCCSnapshot (for instance, the one-XID-cache idea sounds promising); but I think that's material for further research and a separate patch.
Patch attached. Doesn't seem worth a separate thread.
It's not clear to me if anyone wanted to do further review/testing of this patch, or if I should go ahead and push it (after fixing whatever comments need to be fixed).
Push, please.
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
