Re: Hash Indexes

On Wed, Oct 5, 2016 at 10:22 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Oct 4, 2016 at 12:36 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> I think one way to avoid the risk of deadlock in above scenario is to
>> take the cleanup lock conditionally, if we get the cleanup lock then
>> we will delete the items as we are doing in patch now, else it will
>> just mark the tuples as dead and ensure that it won't try to remove
>> tuples that are moved-by-split.  Now, I think the question is how will
>> these dead tuples be removed.  We anyway need a separate mechanism to
>> clear dead tuples for hash indexes as during scans we are marking the
>> tuples as dead if corresponding tuple in heap is dead which are not
>> removed later.  This is already taken care in btree code via
>> kill_prior_tuple optimization.  So I think clearing of dead tuples can
>> be handled by a separate patch.
>
> That seems like it could work.
>

I have implemented this idea and it works for MVCC scans.  However, I
think this might not work for non-MVCC scans.  Consider a case where
in Process-1, hash scan has returned one row and before it could check
it's validity in heap, vacuum marks that tuple as dead and removed the
entry from heap and some new tuple has been placed at that offset in
heap.  Now when Process-1 checks the validity in heap, it will check
for different tuple then what the index tuple was suppose to check.
If we want, we can make it work similar to what btree does as being
discussed on thread [1], but for that we need to introduce page-scan
mode as well in hash indexes.   However, do we really want to solve
this problem as part of this patch when this exists for other index am
as well?


[1]  - https://www.postgresql.org/message-id/CACjxUsNtBXe1OfRp%3DacB%2B8QFAVWJ%3Dnr55_HMmqQYceCzVGF4tQ%40mail.gmail.com

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com