Re: heap_sync seems rather oblivious to partitioned tables (wal_level=minimal)

On 28 September 2018 at 14:25, Amit Langote
<Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> Looking at the patch itself, does it seem like both the newly added
> comments repeat the same point (that we'll need per-partition hi_options
> to enable these optimizations) and are pretty close to each other?

Thanks for looking at this.

I don't agree that we can skip explaining why one of the optimisations
can't be applied just because we've explained why a similar
optimisation cannot be applied somewhere close by. I think that the
WAL/FSM optimisation can fairly easily be improved on and probably
fixed in PG12 as we can just lazily determine per-partition if it can
be applied to that partition or not.

For the FREEZE optimisation, since we ERROR out in cases where it's
requested but is not possible, it does not seem likely we'll ever fix
that since to do that we'd need to determine that all partitions have
just been truncated or were only just created in this transaction.
Since we've both recently done a bit of work in the area of speeding
up COPY, then I doubt either of us would like to go and slow it down
again by adding a pre-check that goes and opens all the partitions
before the copy begins. That's going to have a huge negative
performance impact on small copies to 1 partition when there are many
partitions attached.

So in this regard, you'll notice that the comments are not that
similar. One explains that we could improve on it, and the other
attempts to mention that it would be surprising if we performed a
FREEZE for some partitions but not others.

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David Rowley http://www.2ndQuadrant.com/
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