Re: Hash Indexes

On Wed, Jun 22, 2016 at 5:14 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> We can do it in the way as you are suggesting, but there is another thing
> which we need to consider here. As of now, the patch tries to finish the
> split if it finds split-in-progress flag in either old or new bucket. We
> need to lock both old and new buckets to finish the split, so it is quite
> possible that two different backends try to lock them in opposite order
> leading to a deadlock. I think the correct way to handle is to always try
> to lock the old bucket first and then new bucket. To achieve that, if the
> insertion on new bucket finds that split-in-progress flag is set on a
> bucket, it needs to release the lock and then acquire the lock first on old
> bucket, ensure pincount is 1 and then lock new bucket again and ensure that
> pincount is 1. I have already maintained the order of locks in scan (old
> bucket first and then new bucket; refer changes in _hash_first()).
> Alternatively, we can try to finish the splits only when someone tries to
> insert in old bucket.

Yes, I think locking buckets in increasing order is a good solution.
I also think it's fine to only try to finish the split when the insert
targets the old bucket. Finishing the split enables us to remove
tuples from the old bucket, which lets us reuse space instead of
accelerating more. So there is at least some potential benefit to the
backend inserting into the old bucket. On the other hand, a process
inserting into the new bucket derives no direct benefit from finishing
the split.

--
Robert Haas
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