Re: POC: converting Lists into arrays

I wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
>>> 1. This still involves at least two palloc's for every nonempty List,
>>> because I kept the header and the data array separate. Perhaps it's
>>> worth allocating those in one palloc.

>> Hm, I think if we force external code to audit their code, we better
>> also do this. This is a significant number of allocations, and I don't
>> think it'd be good to spread this out over two releases.

> If we choose to do it, I'd agree with doing both in the same major release
> cycle, so that extensions see just one breakage. But I think it'd still
> best be developed as a follow-on patch.

By the by ... this idea actively breaks the mechanism I'd proposed for
preserving foreach's behavior of evaluating the List reference only once.
If we save a hidden copy of whatever the user says the List reference
is, and then he assigns a new value to it mid-loop, we're screwed if
the list header can move.

Now do you see why I'm a bit afraid of this? Perhaps it's worth doing,
but it's going to introduce a whole new set of code breakages that are
going to be just as hard to audit for as anything else discussed in
this thread. (Example: outer function creates a nonempty list, and
passes it down to some child function that appends to the list, and
there's no provision for returning the possibly-modified list header
pointer back up.) I'm not really convinced that saving one more palloc
per List is worth it.

regards, tom lane