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Re: BUG #6572: The example of SPI_execute is bogus

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-bugs(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org
Subject: Re: BUG #6572: The example of SPI_execute is bogus
Date: 2012-04-13 22:53:48
Message-ID: CA+TgmobS_t4ixo+wEAx-KyhRzapYTapY06q0qSctFuGiOBSEmA@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-bugspgsql-hackers
On Thu, Apr 5, 2012 at 2:39 AM, Hitoshi Harada <umi(dot)tanuki(at)gmail(dot)com> wrote:
> On Wed, Apr 4, 2012 at 8:00 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> umi(dot)tanuki(at)gmail(dot)com writes:
>>> http://www.postgresql.org/docs/9.1/static/spi-spi-execute.html
>>
>>> ===
>>> SPI_execute("INSERT INTO foo SELECT * FROM bar", false, 5);
>>> will allow at most 5 rows to be inserted into the table.
>>> ===
>>
>>> This seems not true unless I'm missing something.
>>
>> Hmm ... that did work as described, until we broke it :-(.  This is an
>> oversight in the 9.0 changes that added separate ModifyTuple nodes to
>> plan trees.  ModifyTuple doesn't return after each updated row, unless
>> there's a RETURNING clause; which means that the current_tuple_count
>> check logic in ExecutePlan() no longer stops execution as intended.
>>
>> Given the lack of complaints since 9.0, maybe we should not fix this
>> but just redefine the new behavior as being correct?  But it seems
>> mighty inconsistent that the tuple limit would apply if you have
>> RETURNING but not when you don't.  In any case, the ramifications
>> are wider than one example in the SPI docs.
>>
>> Thoughts?
>
> To be honest, I was surprised when I found tcount parameter is said to
> be applied to even INSERT.  I believe people think that parameter is
> to limit memory consumption when returning tuples thus it'd be applied
> for only SELECT or DML with RETURNING.  So I'm +1 for non-fix but
> redefine the behavior.  Who wants to limit the number of rows
> processed inside the backend, from SPI?

Yeah.  I think it would be a good idea for UPDATE and DELETE to expose
a LIMIT option, but I can't really see the virtue in making that
functionality available only through SPI.

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

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