Re: Performance difference in accessing differrent columns in a Postgres Table

On Mon, Jul 30, 2018 at 12:11 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
wrote:

> 2018-07-30 1:00 GMT+02:00 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>
>> David Rowley <david(dot)rowley(at)2ndquadrant(dot)com> writes:
>> > On 29 July 2018 at 17:38, Dinesh Kumar <dns98944(at)gmail(dot)com> wrote:
>> >> I found performance variance between accessing int1 and int200 column
>> which
>> >> is quite large.
>>
>> > Have a look at slot_deform_tuple and heap_deform_tuple. You'll see
>> > that tuples are deformed starting at the first attribute. If you ask
>> > for attribute 200 then it must deform 1-199 first.
>>
>> Note that that can be optimized away in some cases, though evidently
>> not the one the OP is testing. From memory, you need a tuple that
>> contains no nulls, and all the columns to the left of the target
>> column have to be fixed-width datatypes. Otherwise, the offset to
>> the target column is uncertain, and we have to search for it.
>>
>
> JIT decrease a overhead of this.
>

The bottleneck here is such a simple construct, I don't see how JIT could
improve it by much.

And indeed, in my hands JIT makes it almost 3 times worse.

Run against ab87b8fedce3fa77ca0d6, I get 12669.619 ms for the 2nd JIT
execution and 4594.994 ms for the JIT=off.

Cheers,

Jeff