Re: UPDATE of partition key

On Mon, Feb 20, 2017 at 2:58 PM, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
> I am inclined to at least have some option for the user to decide the
> behaviour. In the future we can even consider support for walking
> through the ctid chain across multiple relfilenodes. But till then, we
> need to decide what default behaviour to keep. My inclination is more
> towards erroring out in an unfortunate even where there is an UPDATE
> while the row-movement is happening. One option is to not get into
> finding whether the DELETE was part of partition row-movement or it
> was indeed a DELETE, and always error out the UPDATE when
> heap_update() returns HeapTupleUpdated, but only if the table is a
> leaf partition. But this obviously will cause annoyance because of
> chances of getting such errors when there are concurrent updates and
> deletes in the same partition. But we can keep a table-level option
> for determining whether to error out or silently lose the UPDATE.

I'm still a fan of the "do nothing and just document that this is a
weirdness of partitioned tables" approach, because implementing
something will be complicated, will ensure that this misses this
release if not the next one, and may not be any better for users. But
probably we need to get some more opinions from other people, since I
can imagine people being pretty unhappy if the consensus happens to be
at odds with my own preferences.

> Another option I was thinking : When the UPDATE is on a partition key,
> acquire ExclusiveLock (not AccessExclusiveLock) only on that
> partition, so that the selects will continue to execute, but
> UPDATE/DELETE will wait before opening the table for scan. The UPDATE
> on partition key is not going to be a very routine operation, it
> sounds more like a DBA maintenance operation; so it does not look like
> it would come in between usual transactions.

I think that's going to make users far more unhappy than breaking the
EPQ behavior ever would.

--
Robert Haas
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