From: | Fujii Masao <masao(dot)fujii(at)oss(dot)nttdata(dot)com> |
---|---|
To: | "osumi(dot)takamichi(at)fujitsu(dot)com" <osumi(dot)takamichi(at)fujitsu(dot)com>, 'Ashutosh Bapat' <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com>, "tsunakawa(dot)takay(at)fujitsu(dot)com" <tsunakawa(dot)takay(at)fujitsu(dot)com> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: Disable WAL logging to speed up data loading |
Date: | 2020-10-01 03:58:19 |
Message-ID: | 7bedf641-5e5a-09ad-73e3-a3cb9399270e@oss.nttdata.com |
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Lists: | pgsql-hackers |
On 2020/10/01 12:04, osumi(dot)takamichi(at)fujitsu(dot)com wrote:
> Hello.
>
>>>> 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?
> Sorry, my last expression about "to lose data" was not precise.
>
> When unlogged tables are used and the server is crashed unexpectedly,
> all data after the last backup is truncated without WAL, right ?
> In this case, sequential commands for unlogged tables disappear.
To avoid this issue we can mark the table as logged. You may be worried about
that ALTER TABLE SET LOGGED/UNLOGGED would take a long time because
the table needs to be rewriitten. One idea for that is to improve that command
so that it skips the table rewrite if wal_level=minimal.
Of course, also you can change wal_level after marking the table as unlogged.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
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