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

2018-07-30 13:19 GMT+02:00 Jeff Janes <jeff(dot)janes(at)gmail(dot)com>:

> 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.
>

look on
http://www.postgresql-archive.org/PATCH-LLVM-tuple-deforming-improvements-td6029385.html
thread, please.

Regards

Pavel

> Cheers,
>
> Jeff
>