Re: ATTACH/DETACH PARTITION CONCURRENTLY

On 8 August 2018 at 00:47, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2018-08-08 00:40:12 +1200, David Rowley wrote:
>> 1. Obtain a ShareUpdateExclusiveLock on the partitioned table rather
>> than an AccessExclusiveLock.
>> 2. Do all the normal partition attach partition validation.
>> 3. Insert pg_partition record with partvalid = true.
>> 4. Invalidate relcache entry for the partitioned table
>> 5. Any loops over a partitioned table's PartitionDesc must check
>> PartitionIsValid(). This will return true if the current snapshot
>> should see the partition or not. The partition is valid if partisvalid
>> = true and the xmin precedes or is equal to the current snapshot.
>
> How does this protect against other sessions actively using the relcache
> entry? Currently it is *NOT* safe to receive invalidations for
> e.g. partitioning contents afaics.

I'm not proposing that sessions running older snapshots can't see that
there's a new partition. The code I have uses PartitionIsValid() to
test if the partition should be visible to the snapshot. The
PartitionDesc will always contain details for all partitions stored in
pg_partition whether they're valid to the current snapshot or not. I
did it this way as there's no way to invalidate the relcache based on
a point in transaction, only a point in time.

I'm open to better ideas, of course.

--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services