Re: Hash Indexes

On Tue, Dec 20, 2016 at 7:11 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Dec 20, 2016 at 4:51 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> We have mainly four actions for squeeze operation, add tuples to the
>> write page, empty overflow page, unlinks overflow page, make it free
>> by setting the corresponding bit in overflow page. Now, if we don't
>> log the changes to write page and freeing of overflow page as one
>> operation, then won't query on standby can either see duplicate tuples
>> or miss the tuples which are freed in overflow page.
>
> No, I think you could have two operations:
>
> 1. Move tuples from the "read" page to the "write" page.
>
> 2. Unlink the overflow page from the chain and mark it free.
>
> If we fail after step 1, the bucket chain might end with an empty
> overflow page, but that's OK.
>

If there is an empty page in bucket chain, access to that page will
give an error (In WAL patch we are initializing the page instead of
making it completely empty, so we might not see an error in such a
case). What advantage do you see by splitting the operation?
Anyway, I think it is better to discuss this in WAL patch thread.

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