Re: POC: converting Lists into arrays

Here's a v3 incorporating Andres' idea of trying to avoid a separate
palloc for the list cell array.  In a 64-bit machine we can have up
to five ListCells in the initial allocation without actually increasing
space consumption at all compared to the old code.  So only when a List
grows larger than that do we need more than one palloc.

I'm still having considerable difficulty convincing myself that this
is enough of a win to justify the bug hazards we'll introduce, though.
On test cases like "pg_bench -S" it seems to be pretty much within the
noise level of being the same speed as HEAD.  I did see a nice improvement
in the test case described in
https://www.postgresql.org/message-id/6970.1545327857@sss.pgh.pa.us
but considering that that's still mostly a tight loop in
match_eclasses_to_foreign_key_col, it doesn't seem very interesting
as an overall figure of merit.

I wonder what test cases Andres has been looking at that convince
him that we need a reimplementation of Lists.

            regards, tom lane