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Previously, a worker process would establish values for these based on its own start time. In v10 and up, this can trivially be shown to cause misbehavior of transaction_timestamp(), timestamp_in(), and related functions which are (perhaps unwisely?) marked parallel-safe. It seems likely that other behaviors might diverge from what happens in the parent as well. It's not as trivial to demonstrate problems in 9.6 or 9.5, but I'm sure it's still possible, so back-patch to all branches containing parallel worker infrastructure. In HEAD only, mark now() and statement_timestamp() as parallel-safe (other affected functions already were). While in theory we could still squeeze that change into v11, it doesn't seem important enough to force a last-minute catversion bump. Konstantin Knizhnik, whacked around a bit by me Discussion: https://postgr.es/m/6406dbd2-5d37-4cb6-6eb2-9c44172c7e7c@postgrespro.ru
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