Re: Pinning a buffer in TupleTableSlot is unnecessary

On Sat, Nov 12, 2016 at 10:28 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2016-08-30 07:38:10 -0400, Tom Lane wrote:
>> Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
>> > While profiling some queries and looking at executor overhead, I
>> > realized that we're not making much use of TupleTableSlot's ability to
>> > hold a buffer pin. In a SeqScan, the buffer is held pinned by the
>> > underlying heap-scan anyway. Same with an IndexScan, and the SampleScan.
>>
>> I think this is probably wrong, or at least very dangerous to remove.
>> The reason for the feature is that the slot may continue to point at
>> the tuple after the scan has moved on.
>
> FWIW, that's not safe to assume in upper layers *anyway*. If you want to
> do that, the slot has to be materialized, and that'd make a local
> copy. If you don't materialize tts_values/isnull can point into random
> old memory (common e.g. for projections and virtual tuples in general).

So, I think you are arguing in favor of proceeding with this patch?

--
Robert Haas
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