Re: Order of columns in query is important?!

You're right. I misread the problem description.

On Tue, May 26, 2015 at 3:13 AM, Petr Jelinek <petr(at)2ndquadrant(dot)com> wrote:

> On 26/05/15 11:59, CK Tan wrote:
>
>> It has to do with the implementation of slot_getattr, which tries to do
>> the deform on-demand lazily.
>>
>> if you do select a,b,c, the execution would do slot_getattr(1) and
>> deform a, and then slot_getattr(2) which reparse the tuple to deform b,
>> and finally slot_getattr(3), which parse the tuple yet again to deform c.
>>
>> Where as if you do select c, b, a, it would do slot_getattr(3) to deform
>> c, and in the process deform a and b in one pass. Subsequent calls to
>> slot_getattr 1 and 2 would find the attribute ready and available, and
>> return it (without parsing the tuple again).
>>
>>
> If this was the case, changing column order would lead to performance
> increase, not decrease as reported.
>
> My guess would be same as Amits, it's most likely the additional
> projection step.
>
> --
> Petr Jelinek http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>