Re: Hash Indexes

On Tue, Dec 20, 2016 at 9:01 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> 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).

It wouldn't be a new, uninitialized page. It would be empty of
tuples, not all-zeroes.

> What advantage do you see by splitting the operation?

It's simpler. The code here is very complicated and trying to merge
too many things into a single operation may make it even more
complicated, increasing the risk of bugs and making the code hard to
maintain without necessarily buying much performance.

> Anyway, I think it is better to discuss this in WAL patch thread.

OK.

--
Robert Haas
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