Date: Fri, 10 Jul 2015 12:29:02 +0200
From: Andres Freund <andres@anarazel.de>
To: Fujii Masao <masao.fujii@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Martijn van Oosterhout <kleptog@svana.org>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: WAL logging problem in 9.4.3?
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On 2015-07-10 19:23:28 +0900, Fujii Masao wrote:
> Maybe I'm missing something. But I start wondering why TRUNCATE
> and INSERT (or even all the operations on the table created at
> the current transaction) need to be WAL-logged while COPY can be
> optimized. If no WAL records are generated on that table, the problem
> we're talking about seems not to occur. Also this seems safe and
> doesn't degrade the performance of data loading. Thought?

Skipping WAL logging means that you need to scan through the whole
shrared buffers to write out dirty buffers and fsync the segments. A
single insert wal record is a couple orders of magnitudes cheaper than
that.  Essentially doing this juts for COPY is a heuristic.


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