If serialized_snapshot->subxcnt > 0 and serialized_snapshot->xcnt == 0, the old coding would do the wrong thing and crash. This can happen on standby servers.
Report by Andreas Seltenreich. Patch by Thomas Munro, reviewed by Amit Kapila and tested by Andreas Seltenreich.
Branch ------ REL9_5_STABLE
"This can happen on standby servers"
The commit message doesn't mention, but it applies only to parallel query?
Why then is this backpatched to 9.5?
--
Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services