Re: Patch: Write Amplification Reduction Method (WARM)

On Sat, Mar 25, 2017 at 11:24 PM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)com> wrote:
>
> On Sat, 25 Mar 2017 at 11:03 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>>
>> On Sat, Mar 25, 2017 at 12:54 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
>> wrote:
>> > I am not sure how do you want to binary compare two datums, if you are
>> > thinking datumIsEqual(), that won't work. I think you need to use
>> > datatype specific compare function something like what we do in
>> > _bt_compare().
>>
>> How will that interact with types like numeric, that have display
>> scale or similar?
>>
> I wonder why Amit thinks that datumIsEqual won't work once we convert the
> heap values to index tuple and then fetch using index_get_attr. After all
> that's how the current index tuple was constructed when it was inserted.

I think for toasted values you need to detoast before comparison and
it seems datamIsEqual won't do that job. Am I missing something which
makes you think that datumIsEqual will work in this context.

> In
> fact, we must not rely on _bt_compare because that might return "false
> positive" even for two different heap binary values (I think).

I am not telling to rely on _bt_compare, what I was trying to hint at
it was that I think we might need to use some column type specific
information for comparison. I am not sure at this stage what is the
best way to deal with this problem without incurring non trivial cost.

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