Re: Is it useful to record whether plans are generic or custom?

On 2020-07-06 22:16, Fujii Masao wrote:
> On 2020/06/11 14:59, torikoshia wrote:
>> On 2020-06-10 18:00, Kyotaro Horiguchi wrote:
>>
>>>
>>> +    TupleDescInitEntry(tupdesc, (AttrNumber) 8, "last_plan",
>>>
>>> This could be a problem if we showed the last plan in this view.  I
>>> think "last_plan_type" would be better.
>>>
>>> +            if (prep_stmt->plansource->last_plan_type ==
>>> PLAN_CACHE_TYPE_CUSTOM)
>>> +                values[7] = CStringGetTextDatum("custom");
>>> +            else if (prep_stmt->plansource->last_plan_type ==
>>> PLAN_CACHE_TYPE_GENERIC)
>>> +                values[7] = CStringGetTextDatum("generic");
>>> +            else
>>> +                nulls[7] = true;
>>>
>>> Using swith-case prevents future additional type (if any) from being
>>> unhandled.  I think we are recommending that as a convension.
>>
>> Thanks for your reviewing!
>>
>> I've attached a patch that reflects your comments.
>
> Thanks for the patch! Here are the comments.

Thanks for your review!

> + Number of times generic plan was choosen
> + Number of times custom plan was choosen
>
> Typo: "choosen" should be "chosen"?

Thanks, fixed them.

> + <entry role="catalog_table_entry"><para
> role="column_definition">
> + <structfield>last_plan_type</structfield> <type>text</type>
> + </para>
> + <para>
> + Tells the last plan type was generic or custom. If the
> prepared
> + statement has not executed yet, this field is null
> + </para></entry>
>
> Could you tell me how this information is expected to be used?
> I think that generic_plans and custom_plans are useful when
> investigating
> the cause of performance drop by cached plan mode. But I failed to get
> how much useful last_plan_type is.

This may be an exceptional case, but I once had a case needed
to ensure whether generic or custom plan was chosen for specific
queries in a development environment.

Of course, we can know it from adding EXPLAIN and ensuring whether $n
is contained in the plan, but I feel using the view is easier to use
and understand.

Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION