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

On 2018/09/28 12:12, Michael Paquier wrote:
> On Fri, Sep 28, 2018 at 02:46:30PM +1200, David Rowley wrote:
>> 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.
>
> Have you guys looked at what the following patch does for partitions and
> how it interacts with it?
> https://commitfest.postgresql.org/19/528/

Just looked at that patch and noticed that the following hunk won't cope
if COPY's target table is partitioned:

diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 7674369613..7b9a7af2d2 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -2416,10 +2416,8 @@ CopyFrom(CopyState cstate)
{
hi_options |= HEAP_INSERT_SKIP_FSM;

- if (!XLogIsNeeded() &&
- cstate->rel->trigdesc == NULL &&
- RelationGetNumberOfBlocks(cstate->rel) == 0)
- hi_options |= HEAP_INSERT_SKIP_WAL;
+ if (!XLogIsNeeded() && RelationGetNumberOfBlocks(cstate->rel) == 0)
+ hi_options |= HEAP_INSERT_SKIP_WAL;
}

/*

RelationGetNumberOfBlocks would blow up if passed a partitioned table to it.

Applying David's patch will take care of that though.

> The proposed patch is missing the point that documentation also mentions
> the optimizations for COPY with wal_level = minimal:
> <para>
> <command>COPY</command> is fastest when used within the same
> transaction as an earlier <command>CREATE TABLE</command> or
> <command>TRUNCATE</command> command. In such cases no WAL
> needs to be written, because in case of an error, the files
> containing the newly loaded data will be removed anyway.
> However, this consideration only applies when
> <xref linkend="guc-wal-level"/> is <literal>minimal</literal> as all commands
> must write WAL otherwise.
> </para>

I might be wrong but I'm not sure if we should mention here that this
optimization is not applied to partitioned tables due to what appears to
be a implementation-level limitation?

Thanks,
Amit