Re: Pinning a buffer in TupleTableSlot is unnecessary

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).

Andres