Re: ALTER TABLE ADD COLUMN fast default

On 12/06/2017 11:26 AM, Andrew Dunstan wrote:
>> In general, you need to be thinking about this in terms of making sure
>> that it's impossible to accidentally not update code that needs to be
>> updated. Another example is that I noticed that some places the patch
>> has s/CreateTupleDescCopy/CreateTupleDescCopyConstr/, evidently because
>> the former will fail to copy the missing-attribute support data.
>> I think that's an astonishingly bad idea. There is basically no situation
>> in which CreateTupleDescCopy defined in that way will ever be safe to use.
>> The missing-attribute info is now a fundamental part of a tupdesc and it
>> has to be copied always, just as much as e.g. atttypid.
>>
>> I would opine that it's a mistake to design TupleDesc as though the
>> missing-attribute data had anything to do with default values. It may
>> have originated from the same place but it's a distinct thing. Better
>> to store it in a separate sub-structure.
>
> OK, will work on all that. Thanks again.
>

Looking closer at this. First, there is exactly one place where the
patch does  s/CreateTupleDescCopy/CreateTupleDescCopyConstr/.

making this like atttypid suggests that it belongs right outside the
TupleConstr structure. But then it seems to me that the change could
well end up being a darn site more invasive. For example, should we be
passing the number of missing values to CreateTemplateTupleDesc and
CreateTupleDesc? I'm happy to do whatever work is required, but I want
to have a firm understanding of the design before I spend lots of time
cutting code. Once I understand how tupdesc.h should look I should be good.

cheers

andrew

--
Andrew Dunstan https://www.2ndQuadrant.com
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