Re: Implement UNLOGGED clause for COPY FROM

On 2020/07/09 11:36, osumi(dot)takamichi(at)fujitsu(dot)com wrote:
> Hello, hackers.
>
>
> I've been thinking to suggest
> a peformance-oriented feature for COPY FROM.
> It's UNLOGGED clause, which means data loading skipping WAL generation.

This feature can work safely with wal_level=replica or logical?
Or it can work only with wal_level=minimal? If yes, what is the main
difference between this method and wal_skip_threshold?

>
> How to make it work is the following.
> 1. Aquire ACCESS EXCLUSIVE mode to lock the target table and its indexes.
> 2. Mark those relations 'unrecoverable' in pg_class/pg_index.
> 3. Issue one WAL to indicate when COPY UNLOGGED is executed.
> 4. Execute the data loading, bypassing WAL generation for data.
> 5. Sync the data to disk by performing checkpoint.

What happens if the server crashes before #5? Since no WAL for
data-loading can be replayed, the target table should be truncated?

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION