Re: WAL logging problem in 9.4.3?

On 2015-07-06 11:49:54 -0400, Tom Lane wrote:
> One idea I had was to allow the COPY optimization only if the heap file is
> physically zero-length at the time the COPY starts. That would still be
> able to optimize in all the cases we care about making COPY fast for.
> Rather than reverting cab9a0656c36739f, which would re-introduce a
> different performance problem, perhaps we could have COPY create a new
> relfilenode when it does this. That should be safe if the table was
> previously empty.

I'm not convinced that cab9a0656c36739f needs to survive in that
form. To me only allowing one COPY to benefit from the wal_level =
minimal optimization has a significantly higher cost than
cab9a0656c36739f.

My tentative guess is that the best course is to

a) Make heap_truncate_one_rel() create a new relfeilnode. That fixes the
truncation replay issue.

b) Force new pages to be used when using the heap_sync mode in
COPY. That avoids the INIT danger you found. It seems rather
reasonable to avoid using pages that have already been the target of
WAL logging here in general.

Andres