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

On 2020-07-20 11:57, Fujii Masao wrote:
> On 2020/07/17 16:25, Fujii Masao wrote:
>> 
>> 
>> On 2020/07/16 11:50, torikoshia wrote:
>>> On 2020-07-15 11:44, Fujii Masao wrote:
>>>> 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?
>>> 
>>> 
>>> Thanks for your comments!
>>> 
>>> Agreed.
>>> I removed tests on prepare.sql and added them to plancache.sql.
>>> Thoughts?
>> 
>> Thanks for updating the patch!
>> I also applied the following minor changes to the patch.
>> 
>> -        Number of times generic plan was chosen
>> +       Number of times generic plan was chosen
>> -        Number of times custom plan was chosen
>> +       Number of times custom plan was chosen
>> 
>> I got rid of one space character before those descriptions because
>> they should start from the position of 7th character.
>> 
>>   -- but we can force a custom plan
>>   set plan_cache_mode to force_custom_plan;
>>   explain (costs off) execute test_mode_pp(2);
>> +select name, generic_plans, custom_plans from pg_prepared_statements
>> +  where  name = 'test_mode_pp';
>> 
>> In the regression test, I added the execution of 
>> pg_prepared_statements
>> after the last execution of test query, to confirm that custom plan is 
>> used
>> when force_custom_plan is set, by checking from 
>> pg_prepared_statements.
>> 
>> I changed the status of this patch to "Ready for Committer" in CF.
>> 
>> Barring any objection, I will commit this patch.
> 
> Committed. Thanks!

Thanks!

As I proposed earlier in this thread, I'm now trying to add information
about generic/cudstom plan to pg_stat_statements.
I'll share the idea and the poc patch soon.

Regards,

--
Atsushi Torikoshi
NTT DATA CORPORATION