Re: Is it useful to record whether plans are generic or custom? - Mailing list pgsql-hackers

On 2020/07/14 21:24, torikoshia wrote:
> On 2020-07-10 10:49, torikoshia wrote:
>> On 2020-07-08 16:41, Fujii Masao wrote:
>>> On 2020/07/08 10:14, torikoshia wrote:
>>>> 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.
>>>
>>> In your case, probably you had to ensure that the last multiple (or every)
>>> executions chose generic or custom plan? If yes, I'm afraid that displaying
>>> only the last plan mode is not enough for your case. No?
>>> So it seems better to check generic_plans or custom_plans columns in the
>>> view rather than last_plan_type even in your case. Thought?
>>
>> Yeah, I now feel last_plan is not so necessary and only the numbers of
>> generic/custom plan is enough.
>>
>> If there are no objections, I'm going to remove this column and related codes.
> 
> As mentioned, I removed last_plan column.

Thanks for updating the patch! It basically looks good to me.

I have one comment; you added the regression tests for generic and
custom plans into prepare.sql. But the similar tests already exist in
plancache.sql. So isn't it better to add the tests for generic_plans and
custom_plans columns, into plancache.sql?

Regards,


-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION