Re: Hash Indexes

On Sun, Dec 18, 2016 at 8:54 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
>> I committed remove-hash-wrtbuf and fix_dirty_marking_v1 but I've got
>> some reservations about fix_lock_chaining_v1.  ISTM that the natural
>> fix here would be to change the API contract for _hash_freeovflpage so
>> that it doesn't release the lock on the write buffer.  Why does it
>> even do that?  I think that the only reason why _hash_freeovflpage
>> should be getting wbuf as an argument is so that it can handle the
>> case where wbuf happens to be the previous block correctly.
>
> Yeah, as of now that is the only case, but for WAL patch, I think we
> need to ensure that the action of moving all the tuples to the page
> being written and the overflow page being freed needs to be logged
> together as an atomic operation.

Not really.  We can have one operation that empties the overflow page
and another that unlinks it and makes it free.

> Now apart from that, it is
> theoretically possible that write page will remain locked for multiple
> overflow pages being freed (when the page being written has enough
> space that it can accommodate tuples from multiple overflow pages).  I
> am not sure if it is worth worrying about such a case because
> practically it might happen rarely.  So, I have prepared a patch to
> retain a lock on wbuf in _hash_freeovflpage() as suggested by you.

Committed.

-- 
Robert Haas
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