Re: POC: converting Lists into arrays

> On 17 Jul 2019, at 01:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> There are a bunch of places that are using list_delete_first to remove
> the next-to-process entry from a List used as a queue.  In principle,
> we could invert the order of those queues and then use list_delete_last,
> but I thought this would probably be too confusing: it's natural to
> think of the front of the list as being the head of the queue.  I doubt
> that any of those queues get long enough for it to be a serious
> performance problem to leave them as-is.

For cases where an Oid list is copied and then head elements immediately
removed, as in fetch_search_path, couldn’t we instead use a counter and
list_copy_tail to avoid repeated list_delete_first calls?  Something like the
attached poc.

> +List *
> +list_delete_last(List *list)
> +{
> +    check_list_invariants(list);
> +
> +    if (list == NIL)
> +        return NIL;                /* would an error be better? */

Since we’ve allowed list_delete_first on NIL for a long time, it seems
reasonable to do the same for list_delete_last even though it’s hard to come up
with a good usecase for deleting the last element without inspecting the list
(a stack growing from the bottom perhaps?).  It reads better to check for NIL
before calling check_list_invariants though IMO.

Looking mainly at 0001 for now, I agree that the order is insignificant.

cheers ./daniel