[ComputerGuru]

Another option would be a good way of placing indexes on a different physical disk. You could use fast, ephemeral storage like you can for a WAL without amplifying the writes to the same device that is your expensive bottleneck. You could rebuild on data loss.

But it would add complexity to detect out-of-sync indexes and tables.

[SigmundA]

You mean tablespaces: https://www.postgresql.org/docs/current/manage-ag-tablespace... ?

[ComputerGuru]

Maybe? I wasn’t under the impression these could be reliably lost or out of sync without risking data loss?

[SigmundA]

Wasn't aware you could put a WAL on a unreliable storage system either without risking data loss?

Would be interesting for indexes say put them on ram drive and rebuild them on restart if they aren't there just fallback to table scans.

MSSQL has memory optimized tables that do this sort of thing: https://learn.microsoft.com/en-us/sql/relational-databases/i...

[ComputerGuru]

If you lose the WAL you lose the data since the last merge but there’s no risk of corruption. The WAL handles missed syncs fine, too, missing losing just that window of data.

I don’t know if or how Postgres records the transaction number in the index to be able to notice if it’s out of date. If it does, I don’t know of any solution to “catch up” the index besides recreating it, which would be ok if that’s the only issue but from my experience with out-of-date indexes (libc or icu updates, where Postgres doesn’t know if anything IS broken and just reports that it could be), there’s no guarantee you’d even notice and your app could be running completely broken until you rebuild.

[SigmundA]

>If you lose the WAL you lose the data since the last merge but there’s no risk of corruption.

That is not my understanding:

https://www.postgresql.org/docs/current/app-pgresetwal.html

>After running this command on a data directory with corrupted WAL or a corrupted control file, it should be possible to start the server, but bear in mind that the database might contain inconsistent data due to partially-committed transactions. You should immediately dump your data, run initdb, and restore. After restore, check for inconsistencies and repair as needed.

[ComputerGuru]

I guess that depends on the definition of corrupted. It just describes what to do when the WAL is corrupt, but doesn’t say what conditions are considered corrupt. (We use ZFS so I haven’t run into torn writes.)