Re: logical column ordering

On 26.2.2015 23:34, Andres Freund wrote:
> On 2015-02-26 16:16:54 -0600, Jim Nasby wrote:
>> On 2/26/15 4:01 PM, Alvaro Herrera wrote:
>>> The reason for doing it this way is that changing the underlying
>>> architecture is really hard, without having to bear an endless hackers
>>> bike shed discussion about the best userland syntax to use. It seems a
>>> much better approach to do the actually difficult part first, then let
>>> the rest to be argued to death by others and let those others do the
>>> easy part (and take all the credit along with that); that way, that
>>> discussion does not kill other possible uses that the new architecture
>>> allows.
>
> I agree that it's a sane order of developing things. But: I don't
> think it makes sense to commit it without the capability to change
> the order. Not so much because it'll not serve a purpose at that
> point, but rather because it'll essentially untestable. And this is a
> patch that needs a fair amount of changes, both automated, and
> manual.

I agree that committing something without a code that actually uses it
in practice is not the best approach. That's one of the reasons why I
was asking for the use cases we expect from this.

> At least during development I'd even add a function that randomizes
> the physical order of columns, and keeps the logical one. Running
> the regression tests that way seems likely to catch a fair number of
> bugs.

That's not all that difficult, and can be done in 10 lines of PL/pgSQL -
see the attached file. Should be sufficient for development testing (and
in production there's not much use for that anyway).

>> +1. This patch is already several years old; lets not delay it further.
>>
>> Plus, I suspect that you could hack the catalog directly if you
>> really wanted to change LCO. Worst case you could create a C
>> function to do it.
>
> I don't think that's sufficient for testing purposes. Not only for
> the patch itself, but also for getting it right in new code.

I think we could calls to the randomization functions into some of the
regression tests (say 'create_tables.sql'), but that makes regression
tests ... well, random, and I'm not convinced that's a good thing.

Also, this makes regression tests harder to think, because "SELECT *"
does different things depending on the attlognum order.

--
Tomas Vondra http://www.2ndQuadrant.com/
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