Re: Pinning a buffer in TupleTableSlot is unnecessary

On Mon, Nov 14, 2016 at 10:23 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> 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:
>>>> 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?
>
> I don't believe Andres' claim anyway. There are certainly cases where an
> allegedly-valid slot could be pointing at garbage, but table scans aren't
> one of them, precisely because of the pin held by the slot. It would take
> a fairly wide-ranging code review to convince me that it's okay to lose
> that mechanism.

I don't understand your objection. It seems to me that the
TupleTableSlot is holding a pin, and the scan is also holding a pin,
so one of them is redundant. You speculated that the slot could
continue to point at the tuple after the scan has moved on, but how
could such a thing actually happen? Once the scan finds a tuple, it's
going to store the tuple in the slot and return. It won't get control
back to advance the scan until the next time it's asked for a tuple,
and then it has to update the slot before returning. So I don't see
the problem. If in the future somebody wants to write an executor
node that does random extra work - like advancing the scan - before
returning the tuple the scan already found, they'll have to take
special precautions, but why should anybody want to do that?

I'm kind of puzzled, here. Perhaps I am missing something obvious.

--
Robert Haas
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