On Fri, Nov 25, 2016 at 1:39 PM, Marc-Olaf Jaschke <moj@xxxxxxxxx> wrote: > 1. The following presentation uses serializable transaction > isolation level to force a snapshot too old error. > (http://momjian.us/main/writings/pgsql/features.pdf) > Is it possible to get a snapshot too old error with active > old_snapshot_threshold when the only used transaction isolation > level is read committed? Yes. Transaction isolation level doesn't have much impact on this feature. > 2. Does activating old_snapshot_threshold have a negative impact > on peformance? The latest tests, near the end of development, were run by Tomas Vondra and are reported here: https://www.postgresql.org/message-id/067acb3d-80eb-279d-fce0-90e0a36c6aa2@xxxxxxxxxxxxxxx Note that the "immediate" lines are using a setting of zero, which is ridiculously aggressive, and only intended for testing purposes. The lines ending in "-10-rw" are using a 10 minute expiration, which is still far more aggressive than I would expect most environments to use, but should give some idea what the impact could be "worst case". Tomas says in the linked email, "I'm personally convinced the performance impact is within noise." FWIW, it makes no sense to enable this feature with a read-only workload, but that configuration was heavily tested, as it represents a worst case. Also note that Tomas used a multi-socket NUMA machine for these tests, because that is where we saw the largest differences during early tests, before some optimizations were applied. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-admin mailing list (pgsql-admin@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin
