Re: POC: converting Lists into arrays

Hi,

On 2019-03-02 18:11:43 -0500, Tom Lane wrote:
> I wonder what test cases Andres has been looking at that convince
> him that we need a reimplementation of Lists.

My main observation was from when the expression evaluation was using
lists all over. List iteration overhead was very substantial there. But
that's not a problem anymore, because all of those are gone now due to
the expression rewrite.  I personally wasn't actually advocating for a
new list implementation, I was/am advocating that we should move some
tasks over to a more optimized representation.

I still regularly see list overhead matter in production workloads. A
lot of it being memory allocator overhead, which is why I'm concerned
with a rewrite that doesn't reduce the number of memory allocations. And
a lot of it is stuff that you won't see in pgbench - e.g. there's a lot
of production queries that join a bunch of tables with a few dozen
columns, where e.g. all the targetlists are much longer than what you'd
see in pgbench -S.

Greetings,

Andres Freund