At Thu, 1 Oct 2020 10:01:56 +0900, Fujii Masao <masao.fu...@oss.nttdata.com> wrote in > > > On 2020/09/30 12:10, osumi.takami...@fujitsu.com wrote: > > Hello, Ashutosh > > > >> Can they use a database with all unlogged tables? > > Unfortunately, no. They want to switch a cluster condition to "without > > WAL logging" > > only when they execute night bulk loading for their data warehouse. > > In other words, they would like to keep their other usual operations > > with WAL. > > In addition, using all tables as unlogged brings about > > the risk to lose data warehouse's data caused by an unexpected server > > crash or power outage. > > But the same issue can happen even in the proposed approach > because Tsunakawa-san explains as follows? > > > The server refuses to start (pg_ctl start fails) after an abnormal > > shutdown due to power outage, pg_ctl's immediate shutdown, etc, > > showing a straightforward message like MySQL.
Exactly. Crash safety cannot be a rationale for this feature. The proposed feature seems to give an advantage to a massive loading onto a already very large table. Currently turning a large logged table into an unlogged one runs a table copy that takes a long time. After the bulk-loading, to put back the table huge amount of WAL is emitted and a table-copy takes a long time. If we had the wal_level=none feature, the preparation time is not needed and the put-back time can be offloaded to online backing-up. The crash-unsafe period is inevitable but would be shorten by using storage snapshots. On the other hand, a massive loading to an empty table is efficiently performed by wal_level=minimal. As I mentioned in another thread, if ALTER TABLE LOGGED/UNLOGGED doesn't need a table copy (consequently no insertion WAL are emitted), it would work. I'm not which is better, or easier to realize. regards. -- Kyotaro Horiguchi NTT Open Source Software Center
