I have three tables: data, processed, and processing_status. The first one has rows coming in quite quickly. Every few minutes, I check the highest ID in data and compare it to processing_status. If enough new datapoints have come in, the new ones are processed and aggregated into processed table, and processing_status is updated to the highest ID. Obviously, both of those happen in one transaction, so that if there's a crash, everything stays sane, and neither table is updated without the other.
I'd like to improve the performance of this, regarding the bloat to the WAL logs. If I set processed and processing_status to UNLOGGED in Postgres, will I retain the property that when I commit the transaction, either both tables are updated, or neither? Even in the event of a crash?
I believe that if I set UNLOGGED, the difference is that in a crash, it will always revert both, whereas in LOGGED mode, the WAL could be used to include a few extra in-progress transactions right during the crash. Is this understanding accurate?
Best Answer
Your understanding is correct. UNLOGGED tables participate in transactions just like regular, logged tables.
The only difference is the crash-safety, so you may lose data if your server crashes (unexpected power-outage, operating system crash, Postgres bug, ...)