Re: [HACKERS] Enabling parallelism for queries coming from SQL orother PL functions

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От Robert Haas
Тема Re: [HACKERS] Enabling parallelism for queries coming from SQL orother PL functions
Дата
Msg-id CA+Tgmoate8SvGr=3e76ONdNn_16J7Csv0wd7wGxTrnVryY9drA@mail.gmail.com
обсуждение исходный текст
Ответ на Re: [HACKERS] Enabling parallelism for queries coming from SQL orother PL functions  (Rafia Sabih <rafia.sabih@enterprisedb.com>)
Ответы Re: [HACKERS] Enabling parallelism for queries coming from SQL orother PL functions  (Dilip Kumar <dilipbalaut@gmail.com>)
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On Wed, Mar 22, 2017 at 3:00 AM, Rafia Sabih <rafia.sabih@enterprisedb.com> wrote:
On Wed, Mar 22, 2017 at 12:55 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Mar 21, 2017 at 6:36 AM, Dilip Kumar <dilipbalaut@gmail.com> wrote:
>> How about taking the decision of execute_once based on
>> fcache->returnsSet instead of based on lazyEval?
>>
>> change
>> + ExecutorRun(es->qd, ForwardScanDirection, count, !es->lazyEval);
>> to
>> + ExecutorRun(es->qd, ForwardScanDirection, count, !fcache->returnsSet);
>>
>> IMHO, Robert have the same thing in mind?
>
> Yeah, something like that.
>
Done in execute-once-v2.patch

So, let's see here.  We are safe so long as we're sure that, when postquel_getnext() returns, postquel_end() will be called next without iterating the loop in fmgr_sql().  That will definitely be true if fcache->returnsSet is true.  It will also be true if postquel_getnext returns true, which will be true whenever count == 0, which will be true whenever es->lazyEval is false.

So couldn't we actually make this test !fcache->returnsSet || !es->lazyEval?  That would let us allow parallel execution for all non-set-returning functions, and also for set-returning functions that end up with es->lazyEval set to false.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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