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

On Wed, Sep 5, 2018 at 12:00 PM Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

> On Wed, Sep 5, 2018 at 12:21 AM Dinesh Kumar <dns98944(at)gmail(dot)com> wrote:
>
>> Hi All,
>> I was wondering whether the case is solved or still continuing. As a
>> Postgres newbie, I can't understand any of the terms (JIT, tuple
>> deformation) as you mentioned above. Please anyone let me know , what is
>> the current scenario.
>>
>>
> JIT is a just-in-time compilation, which will be new in v11. Tuple
> deforming is how you get the row from the on-disk format to the in-memory
> format.
>
> Some people see small improvements in tuple deforming using JIT in your
> situation, some see large decreases, depending on settings and apparently
> on hardware. But regardless, JIT is not going to reduce your particular
> use case (many nullable and actually null columns, referencing a
> high-numbered column) down to being constant-time operation in the number
> of preceding columns. Maybe JIT will reduce the penalty for accessing a
> high-numbered column by 30%, but won't reduce the penalty by 30 fold. Put
> your NOT NULL columns first and then most frequently accessed NULLable
> columns right after them, if you can.
>

Correction: NOT NULL columns with fixed width types first. Then of the
columns which are either nullable or variable width types, put the most
frequently accessed earlier.